Many people borrow things from others. Have you ever stopped to think what might happen if you forget to return something that you borrowed? This is a question that usually won't come up until it actually happens.
In the vast majority of cases, a person who loans out something will ask for it back if it seems as though the borrower has forgotten to return it. The borrower will likely return it and all will be well. It isn't often that there is a possible criminal matter at play in these cases.
One thing that you have to remember is that in order to be convicted of theft, you have to have willingly and purposefully chosen to deprive the rightful owner of their possession. This means that you can't accidentally steal something.
It is always best to return items as soon as you are done with them. This takes away the risk of you being accused of stealing the items since you won't have them in your possession for very long.
When you are facing a theft charge, remember that the burden to prove that you committed a crime falls on the prosecutor. If there is any reasonable doubt about your actions, the jury shouldn't convict you. In the case of an unreturned borrowed item, the prosecution might have difficulties proving this point.
Even if you think that you aren't going to be convicted, you should still find out what options you have and what penalties you might face. This can help you decide exactly how you will handle the matter.