Seriously, though, a jury's going to side with a witness if they can't be proven wrong.
Not necessarily. There are a lot of ways to attack a witness even if you can't directly disprove his account of events. You could say he had an inadequate opportunity to observe (maybe he was mugged at night and wouldn't have been able to see the guy's face clearly; maybe it was a drive-by shooting and the witness fled as soon as he saw the gun -- did he really get a good look at the shooter's face?). You could say the witness had a motive to lie (maybe the defendant is a member of a rival gang; maybe the witness was a police officer who might have been covering up his own misconduct). You could say the identification procedures used by police were overly suggestive (it was a "show-up" of one guy in handcuffs in the back of a police car, rather than a more neutral lineup of 5-6 guys).
(Having said that, a lot of defendants are convicted purely on eyewitness testimony, with no physical evidence linking them to the crime. I'm generally suspicious of these cases when it's an identification of a stranger with limited opportunity to observe. But that's how it is. I guess a lot of jurors think like you -- they put too much trust in eyewitness testimony if it's not "disproven.")
To be fair, in practice, those are ways to disprove the account, so you guys aren't really disagreeing. ---
Seplito Nash, Smelling Like the Vault since 1996 Step FOUR! Get Paid!