Once upon a time I worked for a company that had the top executives constantly on the road, traveling, flying to conferences and meetings with clients. These folks struggled to keep up with their expenses, and really suffered when it came time to filing company travel expenses. I often heard these executives complaining, and one time I hear one executive explain to another on how to pad his expenses, and make a nice stipend that padded his wallet. This was fraud, and eventually it caught up with him, and he got fired and had to reimburse the company for his expense fraud.
My friends over at Tallie know about expense fraud since they are in the business of building software for filing and managing business expenses. Read their take on Expense Fraud.