A 1-point safety would be a blocked PAT that the defense accidentally kicks or knocks out of the back of the endzone after it is blocked. In the NFL, the defense can't advance a blocked PAT, but if the ball ends up going out of the back of the defenses endzone and they were the last to touch it, the kicking team is awarded a 1 point safety. Hasn't happened in the NFL for like 70 years or something like that.

A little more common in the NCAA because the defense can advance a blocked PAT. Happened a couple of years ago in the Fiesta Bowl with Oregon vs Kansas State. KSU blocked the Oregon PAT, recovered the ball, backtracked into their own endzone and got tackled. 1 point to Oregon.