Two red ctf bases next to each other (or a red and a blue ctf base) will kill you in single player mode. This is because you default to blue team, and capturing the flag in SP will kill you for some reason. However, you won't lose a life.

In order to lose a life, you have to take damage. When you capture the flag, the screen blacks out. At this point, any damage you take will cause you to lose a life. I believe this is because you are at 0 health during the black screen period, so taking damage basically kills you a 2nd time (but for real).

I used this technique for regular death pits in my levels, since I wanted all of them to work without requiring JJ2+. Thus, you fall in a pit -> hit a tube that delays you (so you stop blinking and can take damage) -> capture the flag -> hit a bees event that causes you to lose a life. Other people have used different systems (blur did this with spike balls, I believe) but it's all the same principle.

Note that this means the decontamination rooms in that level CAN make you lose a life. This will happen if a float lizard chases you in, and hits you just at the right moment (i.e. immediately after your screen goes black). This is very rare though, so I never worried about fixing it.