I might be wrong, but I think I accidentally stole my friend's potato chips.
Are you serious? How could you accidentally steal potato chips?
I think I picked up their bag of chips by mistake.
It's good to be considerate enough about others to realize you might have done something wrong. If you stole their chips then you should return them promptly. Otherwise your friend might wonder what happened to their chips and get annoyed.
You're right, I'm going to give them back right away.
That's a good idea, because it isn't right to keep things that aren't yours. I'm sure your friend will be pleased to get their chips back.