For me it's stupid audit stuff. The date entered as the transaction is a date sooner than it posted and stupid stuff like that. I've mostly trained myself but I still get stuff bounced for little things like that. I've mostly trained myself to follow all the nit-picky rules carefully but I still miss from time to time. As I say, it's mostly not Concur itself which usually warns you of missing fields/receipts.

For me the real pain was paying vendors or freelancers. They all had to be input as an additional vendor, which is reasonable, but there were dozens of fields within each vendor, many of which were esoteric in they're labeling (likely a fault of implementation on our side). Then I would get a rejection via email that a particular field wasn't filled out (why wasn't it a required field?), and it was often hard to find where to input that field -- very unintuitive. I think I even had to go through a company admin once to add an associated email address to a vendor.

Beyond that, just the philosophy of creating a "purchase order" to pay a UX researcher who did 10 hours of work over three months seemed goofy -- purchase order is what I think of when doing something like ordering printer paper, not getting an invoice paid.