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Individual variations, one might say aberrations, can be enormous in Tachinidae. To find flies with left and right with different number of bristles is quite common.
Some variations are known to be common in some species, ie. for instance Oswaldia muscaria with an O. eggeri-like number of bristles on the thorax. With a simple key, one definitely goes wrong, while put together in a box the difference is more than obvious to the naked eye.
Extreme examples are in my experience a Nowickia ferox with a very long petiole in its wing venation. Another source of aberration are intersexes (not that rare at all !)
So, message: don't bother too much about one individual aberration. It is annoying, but they occur.

If you have two specimen with the same aberration, that is another matter.

In Gonia, in my experience the presence of light pruinescense and the extension of this, is quite stable feature. Therefore, I rely on this more than on the shape of reddish sidemarkings or black vitta.

And in the end, I am satisfied with 95 % reliable identification from pictures. Really 100 % is not possible without specimen on your desk.


Theo