According to Jaap (http://www.geocities.com/jaapsch/puzzles/cube2.htm) a 3 move solve would happen every 321 out of 3,674,160 scrambles (about 1 in 11446). A 5 move solve would be 9,992 out of 3,674,160 scrambles (about in in 368).
Oh, I get it. I have had about two or three last layer skips ever, one was around 2 or 3 seconds, the other two were under 2 seconds. The two that were under two seconds had about a 4 move first layer, so they were really easy, the other one had a normal first layer. That was when I used a regular Fridrich method though, and now I use Ortega so it is less likely to happen.