One big mistake I've read about is cutting a steering tube too short. That's usually the result of measuring all the items in the stack and adding wrong or forgetting to include the spacers or stem length. I assemble the bike as I intend to ride it, then scribe a mark at the top of the stem, cut slightly above that mark and then place a 5mm spacer on top of the stem. If there's any doubt about the bar height, leave additional length and plan on recutting later. It's not hard to do, even with the fork still in the frame.