I did this same thing some years ago. I was in such a hurry and hadn't noticed a gate change announcement. The GA just tore my boarding stub and since I was one of the first to board my seat was empty. I sat down and made myself comfy until the first announcement that we were headed for ORD (I was heading for LAX!). At first I thought it was one of those common mistakes where the FA comes back on the speaker and says "oops, haha, no, we're going to LAX" ... but the second time the announcement was made I realized I'd better ask. Needless to say I felt humiliated when I realized I was on the wrong plane!