I'm quite confident there is a short and to-the-point idiom to express the idea of someone who is an 'expert' in his field, suddenly is found in a situation where he is ignorant. Something that would sound like, or express the idea "the correcter stands corrected". Like saying: "A-ha! You who are a linguist and always correcting everyone's spoken and written language, look at the mistake you just made!".
Of course, the idiom would be an expression of that idea so it doesn't have to be language-related as I've explained here.