Easy. Look back at your employment contract. How much did they say they pay you? If they overpay you, means you have to give them back cause it clearly a "glitch". If you try to bring this to court, they will still win cause it's stated in the contract. Also for employer relation sake, I suggest you to pay it back or you will face future problem even with new company if they get hold of the news.

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Added on June 8, 2010, 2:13 pm

Ya but in a more serious case, a bank counter lady wrongly input an extra "0" into the account of person A. Now that person A found $$$$$$ in his/her account, took it and ran away overseas hoping they cannot trace him/her there. Unfortunately bank call interpol and he/she was caught and put in jail. Well, no one ask the guy/lady to put extra "0", but person A still goes to jail regardless for stealing.