Dubious Punctuation - Careful with those Polygonia
   
The butterflies in the genus Polygonia are characterized by a silvery/white mark in centre of the ventral side of their hind wings.  These markings have earned them common names like "Question Mark" and "Comma". In the case of the Question Mark, the silvery mark is usually in two pieces - a hook and a dot, which superficially resembles the eponymous punctuation mark. In the various comma species, the two pieces are usually fused to form a "V" or "C" shape, sometimes with barbs at the ends.  Examples can be found on these pages:

The authors of some field guides use detailed descriptions of the shapes of these markings as field marks for distinguishing one species from another.  They do work as field marks most of the time, but what the field guides usually fail to mention is that there are exceptions to these "rules". Perhaps it goes without saying that no field mark works 100% of the time, but it is not unusual for some people to latch onto field marks and treat them as though they are not subject to variation.
  
My own observation has been that these markings can be variable within a species. In particular, it appears that the markings are often not well formed on female Polygonia.  In addition, the ventral wing pattern of females is often different from that of males.  The colour is often more even, with less variegation.  Combining these two factors can trip up those who focus too much on the shape of the comma mark and/or the superficial appearance of the ventral hind wing colour/pattern. 

While looking at observations on reporting platforms like eButterfly, iNaturalist, and BugGuide, I have seen two categories of identification errors occurring with members of this genus: 
  1. Comma species are mistaken for Question Marks
  2. One Comma species is misidentified as another

In the first case, the error occurs because the silvery mark on the ventral hind wing of a comma is incomplete or "broken", making it look like the mark on the Question Mark.  Here is a typical example of a pale/winter form Question Mark:

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The error seems to happen chiefly with Eastern Commas ( I believe all the examples I've ever seen have been Eastern Commas ).  Here is an example where the mistake could occur:

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A quick look at the silvery mark on the ventral hindwing might lead one to conclude that this is a Question Mark, when in fact it is an Eastern Comma ( pale/winter form ).  The broad, square-ish shape of the hind wing indicates that this is probably a female.  Note the even reddish brown tone to the wings, and the overall lack of variegation compared to the sample photos on my Eastern Comma page ( which are almost certainly males ).

I've seen a number of examples of this phenomenon in my own photos and in those of other observers.  In contrast, I believe the converse - where the broken mark of a Question Mark is fused together to form a comma-like mark - occurs very rarely.  I have only one example in my own collection ( of course, we see a lot more Commas in Eastern Ontario than we do Question Marks ):

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To be fair, the opportunities for confusion are not confined to the shape of the silvery mark on the ventral hind wing.  Another field mark used for the Question Mark is the presence of an "extra" dark spot on the dorsal fore wing, towards the apex.  In rare cases, this spot may be present on Eastern Commas. Here is an example where the extra spot is faint, but definitely present:

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To emphasize that this is in fact an Eastern Comma, here is a view of the ventral side of this individual:

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Again, note the square-ish shape of the wings, and the relatively even tone of the colour on the ventral side, indicating that it is likely a female.

The second category of identification error that seems to happen frequently with Polygonia occurs when some other comma species is mistaken for the Gray Comma.  The Gray Comma typically has a comma mark that is a simple "V" shape with no barbs:

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  Thus, Gray Comma becomes a sort of default ID for any Polygonia that has a simple "V" shaped comma mark and an even colour to the ventral hindwings.  

In my experience, individuals of other comma species sometimes have partial comma marks that are missing their characteristic barbs.  In other words, other comma species can bear a comma mark that resembles that of the Gray Comma.  When this occurs, someone who is making their ID based solely on the shape of the comma mark can be led astray. Since this malformation seems to occur mostly with females, and females often seem to have a more even toned venter, the problem is compounded.

Here are a few examples where the error could occur:

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This Green Comma ( "Chocolate morph" ) was roadkill I found recently. The small and faint comma mark could easily be mistaken for that of the Gray Comma.  In this photo, the green markings are unmistakable, but with different lighting, they might not be so prominent.  With the even brown tone of the wings, a photo that doesn't show the green spots very well could easily result in a mis-identification - and Gray Comma would be the likely ID that would be chosen.  

An example where this is more likely to happen is show below:

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In this example, the comma mark appears to be virtually absent, and the green spots are barely visible. The even brown colour is superficially similar to the look of the Gray Comma, but it lacks the fine striations that are characteristic of that species.  Fortunately, I have a dorsal shot that shows that it definitely isn't a Gray Comma. Even there, familiarity with the female Green Comma is required, since it doesn't look exactly like a typical (male) Green Comma.

Other sorts of malformation are possible.  Here is a Green Comma that is missing part of the end of the comma mark, making the barb look more like a prong:

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The appearance of the comma is more typical on the butterfly's other wing:

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Here is an example of an even more strangely formed comma mark on another Chocolate Morph Green Comma:

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Again, note that in different lighting, those green spots might not be so easily visible against the dark brown background.

Here is an example of a female Satyr Comma with a truncated comma mark:

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In this case, one would hope that the pale reddish-gold colour would prevent it being mistaken for a Gray Comma, but the photo was taken in bright sunlight.  Under different lighting, the colour might not be as obvious.

The Gray Comma can also suffer this same abbreviation of the comma mark.  I have several examples in my collection, but here is one where the comma mark is reduced to barely more than a dash:

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Where does this leave us if we cannot rely on the shape of the comma mark?

As with any field mark, we should not apply it too slavishly.  We should also consider wing shape, colour, and other markings ( including those on the dorsal side when we can see it ). Most importantly, I believe we should look at the appearance of the butterfly as a whole, rather than focus on a series of separate features.  The best "features" aren't necessarily itemized in any field guide because they are often difficult to describe.
Of course, we should also factor in habitat, flight season, known range, etc.