@Sic - by biting off more than you can chew. Doing a poor job and having to constantly redo spots that don't look as good as the rest. Cutting up and painting a chair rail, doing a horrendous job of merging the pieces. Screwing up the masking so you get paint where you're not supposed to; having to redo those spots. Having a ladder that's *almost* long enough to get those hard to reach spots.

It ended up taking about three weeks to finish the one room when everything was said and done