What makes settling trip expenses harder than it should be?

I built CoolMeow for trips where one person pays for the room, someone else covers dinner, and the details get lost in chat. It keeps expenses, notes, and photos together; personal bookkeeping is free, while a shared ledger needs Basic.

For your last group trip, what caused the most friction: remembering who paid, splitting uneven costs, chasing repayments, or something else? I'd like to use real examples to improve the workflow.