We lost a few people because they moved out of the District of Columbia.
Commit a crime.
Get arrested for a fight where assault charges come into the mix. I do not recommend this.
Have an employer who will bug the court about getting you back.
Have some serious health problems where you can't sit for any length of time. This is another thing I really don't recommend.
Being a jerk or a crazy person does not necessarily get you removed from the jury, it just makes everyone else very uncomfortable or miserable. I seriously do not recommend this.
Maybe this depends on the foreman, but we did take vacation days, visits to the doctor, and other days off so very few of us sat for every single day the jury sat. The foreman had a calendar several months out to track who was going to be out and to insure that there would be 16 or more people scheduled to come in to hear testimony. So we weren't completely stuck, but I think that depends on the foreman and how many people want to take month long vacations.