Condoms failing

Ok, strange thought - condoms are said to fail 2% of the time (when used perfectly). So what kind of failure are we talking? Leaks? Breaks? Leaks over the top of the condom? Slips off?

Note: NOT trying to get pregnant, always use condoms (as I am not on birth control), if he doesn't have them, I always am packing.

Just late night ruminating... I am a result of a broken condom... Curious if that's what the 2% failure means...