One way to screw hourly billing: you feel compromised to stay in a project until it's closed, but the project hasn't a close date, so it might never be properly closed. But you might actually want to get out of it to do some other interesting or more expensive work, and if your client feels like it, he/she might never really free you from your project. That's one danger of hourly billing for hourly beginners.

My first "pro" hourly gig ended up going week-to-week with no end in sight. We were doing a great job, but the VP who was paying us was fighting with another VP over who owned the program. After 2 months I had enough and left. A year later the team was still there.