I have a bronze medalist from back in the day when I worked for a telephone systems company, my sister had a gold at a bank.
Bronze: Sales rep decides to look for another job. He makes up a package, consisting of his resume, a partial listing of our company top customers, and a letter that promised, "If you hire me, I'll bring the complete list!" He then proceeds to send out his package to all of our major competitors using company stationary, company postage, and he told the mail room clerk to mail it for him. He was fired one hour later, and was only allowed to walk out with his personal belongings.
Gold: Manager Trainee is Just.Not.Getting.It. Major mistake after stupid major mistake. One day, one of her customers brings back a money order the Trainee had processed for her. "I wanted a $40 money order, and this is wrong." The money order was for $51,219.89! So my sister is fixing the mistake, the trainee is crying, "I don't what my problem is!" Finally, they let the trainee leave early because she was so distraught she was useless. After she left, my sister finally figured out where the goofy dollar amount came from: it was that day's date (May 12, 1989)! A little while later, someone goes to file something in the Trainee's desk drawer, and that's when they find 2 empty fifths of vodka. Hence the source of all the stupid errors. The next morning, when she came into work, HR was waiting for her. At least they caught her before she caused the bank serious losses.