Designing a military ship (that doesn't spend most of its life in a hangar) with a Max Repair value that's larger than it's MSP (Maintenance Supply Point) value.
I wasn't even a newbie at the time, and that was one of my basic checkpoints when critiquing other peoples' ships on the forums. Still, I wasn't paying attention to my own design, so my destroyers started conking out during task force training. There's an old line that comes to mind about pointing out the speck in your brother's eye while ignoring the log in your own.