My favorite mistake was planets that have hit 75 society level, but have rubbish population. Must have used those ones for arc-jumps early on.
Ahh, I've only started to do that recently, so I can expect some crap planets later on? Awesome >.< I didn't think they'd have a really low population though. By the time you can build an arc, you'd only be losing about 8% population, so it's not a great loss considering you get ~21% increase per turn.
