I'm going to tell the cashier I saw someone shoplifting multiple packs of bacon
When did you see this happen?
I didn't.  I just want to see how stupid people can be when they believe anything they hear.
It is wrong to make up stories. This one could really get another person into trouble.
Maybe, but why not just have a laugh about it.
Practical jokes can be fun at times, but this could have serious negative consequences.  You could cause someone to be arrested solely for the sake of a joke, and that would not be funny or acceptable to anyone.