Just to clarify something: I am talking about Online heads-up. (I believe in real life this trick is not used that frequently).
In online heads-up tournaments, you barely know how your opponent plays, so when your opponent makes his first All-in, is a tough decision.
Lets suppose this scenario happens and you decide to fold, then the opponent shows his cards and shows trash (not even a pair).
Because he showed us the bluff, one could easily get angry and call a second All-in in the future with middle pairs or similar.
So my questions is:
How usual is it that he modified his play and will only do the same trick again with the nuts?
Note: This trick is different than the How to deal with people who randomly go all-in?