Years ago, I had a few "coupon booklet"-type tickets that were seriously discounted such that an individual roundtrip exceeded the cost of three roundtrips or something like that. When I submitted the tickets' expenses after all the trips were used for work, it got flagged since the ticket expense was being submitted nearly a year after the expense was incurred and nearly the same amount of time since the first flight was taken. (I submitted it the morning after the last "coupon" was used.) Apparently accounting doesn't like it when costs are assignable to previous quarters and to different projects (even where the applicable projects requiring such trips were not assignable to external clients). (And he knew I saved beaucoup money since others on the same trip had paid more for airfare than me on those very flights.)