Overconfidence/self-importance? There's no doubt a more formal fallacy as well, but that might not necessarily be the right way to think of -- it sounds like:

(a) I think/have seen X
(b) You think/have seen contradictory thing Y
(c) I'm right
(d) Therefore you're wrong

Without considering that they might be as likely to be mistaken as you are, or the relative certainties of the observations. Although I might have got the wrong end of the stick -- did any particular examples inspire this frustration?