After bone-headedly installing a piece on page 1 of the manual that completely buggered up the car assembly on page 19 (and eventually fixing the issue with judicious disassembly/reassembly) you'd have hoped I'd learned my lesson. Check once, twice and a third time to be SURE before snapping the piece on. Because, unfortunately, the kit design is such that often times it still goes together even with a piece reversed. The pain and agony won't bite you until later when step 247 can't occur because the needed groove/hole/slot isn't where it's supposed to be.
Yes, you'd have thought I couldn't make a doofus mistake IMMEDIATELY after fixing the prior mistake. But ... you'd have been wrong. Doh!