In a perhaps controversial statement, I'll add that I actually like it when people do that, for a few reasons:
Firstly, getting the cheeky reason out of the way, it helps boost the site's number-of-answers-per-question statistic. Too many identify-this questions get one answer, it's accepted and that's that. But still, this is a fairly bad reason, so onto my main one...
Secondly and more importantly, many users come to the site and ask for help identifying a movie. They can't remember what is it, but they know what it's not (e.g. this question), or what it reminds them of (e.g. this question). Now, if there are two or three movies which could fit their description, I don't mind all of them being included. In my mind, that means that in future if a different user is searching for one of the other films, but is using search criteria that matches the answer given to the OP, they'll stumble upon this site and see the accepted answer (which for them will be wrong) and two other answers (which for them could be right).
In effect I'm arguing that allowing this practice, whilst not particularly helpful to the OP who has already accepted an answer, could be very helpful to future users looking for a similar movie.
Of course, not all questions should be treated like this. Some are too vague and could have too many possible answers. But if a detailed and useful answer is added (e.g. the sort @Walt usually provides) which could reasonably be considered the accepted answer if an answer hadn't already been given, I'd be happy leaving it.
On a similar note, that's why I think that people shouldn't delete their answers from these questions when the OP comments that it's not the right movie. If it fit their question in the first place, leave it - it could be helpful to others in the future.