First - large engineering consulting firm and engineers "on assignment" were allowed to travel home every two weeks. They were purchasing airline tickets, submitting expense reports, not traveling, and then getting refunds from the airlines. So getting reimbursed and getting refunds. Finance clerk making minimum wage was doing a routine followup with the airlines and found out they weren't traveling. Many people fired. This was pre-9/11, so much more difficult to do today.

Second - this also was a long time ago, but a guy worked for me on assignment out of town (central PA). Timesheets showed he was observing construction at the site on weekends, but the phone records (we had cards to charge phone calls to), showed he was making phone calls from Atlantic City at the same time. Started paying a little more attention and found him not on site when he said he was. Contractor was covering for him. Both of them got fired.