I once brought a carry on full of random electronics/cables, including a partially disassembled desktop power supply in a "custom made modular plastic enclosure" (read: box made out of Legos, complete with a Lego door for the cables to pass through). To make matters worse I was in a rush while packing so the cables were a huge tangled mess.

They flagged it for further inspection, unsurprisingly, but the agent was pretty amicable, I didn't really get any grief