The Rabbi Heckler is a staple of every Young Israel . There is always one guy who has to pick out the one thing in the Rabbi's speach that disagrees with something he has always known to be true (although he has no reliable source to back it up). He must speak up (usually in front of the entire congragation) and make it known that he knows a thing ar two about this particular topic.
The Rabbi will often answer him by telling the RH to speak with him privately, so as not to embarrass him publically. There are Rabbi Hecklers who go over to the Rabbi privately by themselves. They generally save their burning questions for when the Rabbi is done with his sermon. These are more difficult to spot, particularly due to their timing. The only time they are known to others is when those others have a shayla they need to ask the Rav and have to stand beside the RH until he is done with his ramblings.