Anyone can make the honest mistake of forgetting to return a borrowed item. Time passes, memory slips and before you know it, the person from whom you borrowed the item will wonder whether he or she will ever get their goods back.
Even if the owner is miffed about you keeping something for too long, your actions do not amount to theft if you simply, honestly forgot to return the item.
But when does borrowing become stealing?
Intent to Deprive Permanently
Crossing the boundary from borrowing to theft turns on intent. In fact, it's generally the intent element where most of the legal challenges to theft crop up.
From a legal perspective, in order to be guilty of stealing, you need to have the intent to never return the item to its rightful owner at the time you begin borrowing the item.
If you legitimately forgot to return a borrowed item to its rightful owner, then you lacked specific intent to steal the item. As with other specific intent crimes, much stronger and more credible evidence is required in order for the prosecution to establish guilt.
But if you did indeed "kinda sorta" plan to keep the item when you borrowed it, then that can count as thievery.
The Burden of Proof
Because theft is a crime, prosecutors must prove beyond a reasonable doubt that you had no intention to return the borrowed item and in fact planned on keeping it. For example, a text message or Facebook post bragging to another person that you intend to keep an item you supposedly "borrowed" could be used as evidence of theft.
Even if you did intend on keeping an item under the pretense of borrowing it, the burden of proof often is too difficult to overcome.
As we learned from the Broke Bike Thief, returning stolen property doesn't usually work as a defense to theft -- but it could work wonders when it's a case of borrowing accidentally gone bad. So if you're charged with theft and you give the allegedly stolen item back, then it's entirely possible to get a plea deal or a reduction in the charges.