What crime have I done?
Keeping money that doesn't belong to you &/or you didn't earn. I suppose it would be classified as some sort of fraud.
Ex: Let's say my paycheck is $1000.00 a week. However, one week my employer sends me a check with the decimal point moved to the right two spots so I get a check for $100000.00 by mistake. Your employer would not let you keep the extra 90K & they would want it back. I think there was a story within the past year of someone going to jail for an error similar to that.
In a nutshell if you spend the money, try to keep it, &/or don't pay it back you're fucked. I know in Michigan if you steal more then 10K it's a felony that could lead to 10 years in prison.