Buffy... Not sure that Adam seeing through Jonathan's spell counts, as it's not due to a feature that slips through or anything. Also, which vampire did she 'make out' with and not realise he was a vampire?

The folklore example with demons and vampires having bird feet they couldn't conceal - more Red Right Hand?

I'm not sure the Odin example should be there at all as he is 'usually unable' to hide his missing eye...