This is probably my least favorite situation in magic. You pull off a trick perfectly, you reveal the chosen card, and the spectator believes they chose a different card. Not a heckler, just an honest mistake. What are your outs for this situation.
Sloppy magic isn't fun. Not for you nor for the audience. There are a few things I'd do to improve your chances of avoiding the egg on your face should a trick go south such as described above.
First, I always use witnesses. Anytime a card is selected I always make sure multiple people get a chance to see the selected card. That way, if the spectator forgets or misremembers someone else in the audience if not their party can (and usually will) correct them.
Second, I am ALWAYS certain to make clear that they should remember the card selected. I've used a stock line now for a few years and it usually gets a laugh. I don't mind sharing it because, as I said; it's not my original line. "Make sure you don't for get the card you selected. If you forget, it's going to be a great trick, but a lousy ending!"
Audience control is important. If you do a good job of taking your time, establishing yourself, and what you are doing, you'll cut down on the chances that the mark will forget the card selected in advance. If they do forget the card, chances are good they weren't that involved in you and what you are doing to care enough in the first place.
Sometimes a mark will
know the card revealed is their selected card yet they'll try to play coy with you and act as if you are wrong. This can be frustrating. You can try to call them out on their lie by asking if you are certain its their card, read their body language for response, or poll their friends who were watching. Sometimes you just need to roll with it, and move on, just don't use that same mark in the next effect.
All else fails, have an index or invisible deck in your pocket. If you "miss" ask them what they thought their card was, and then produce it from the aforementioned prop. Not only will you look good for finding their correct card in the first place, and amazing their friends watching, BUT you get a double reveal by also nailing the card they -thought- they selected. ...Now that I think about that, I like it! I just came up with that on the fly and I think I may work that into my act as a backup should something go wrong. ...maybe even stooge it. Hmm.