Last time this happened to me, I pushed and pushed to get a receipt for my client expenses and after weeks of going to and fro I ended up with a letter that simply stated I'd paid X for a ticket from A to B. Looked like a one-off kind of thing rather than the usual c&p form letter, so I suspect someone just decided to do the minimum to stop the debate. As my client pointed out, I could have easily knocked up something more credible in about 10 minutes.

Seems crazy that sometimes you just can't get a receipt for which can be a sizeable purchase. Luckily I had an understanding client. I can think of a couple of others who would have simply refused to reimburse as I was unable to submit documentation as per my contract. (Yes, I do sometimes have unreasonable clients, c'est la vie).