Peter peels off his mask to reassure a young kid he's rescuing, in one of many occasions in the film in which he reveals his secret identity. Fortunately, this little nipper will be too traumatised to remember it for long.
Why It's Key:
By the time the film has run its course, a fair few people have discovered Peter's alter-ego, including Gwen, Captain Stacy, Curtis Connors, his shadowy visitor, and possibly even Aunt May (although the latter is only implied). Not so secret an identity, is it?