The only time you can cancel the spear (or any hit limit move) one hit beyond its legal limit is when your opponent has frame advantage. This is randomly chosen at the start of the round by the CPU, and lasts for that round. Theoretically, there is a chance you could have frame advantage every round of a match. This extra frame causes a frame delay glitch in the input of the player at disadvantage, enabling frame cancels for moves otherwise not possible. The best way to test this is to start a two player game with say Human Smoke and Scorpion, stand them about a character width apart and press LK at the same time on both controllers. If you can judge by eye that both attacks started at the exact same time, then whoever lost can glitch cancel, and you can practice with that character. Even jump kicks can be glitch canceled if timed right. The best way to try this is, HK, LP, JK, TP, aaHP, spear. You can learn the timing for it, it's very reliable. I use to think that character neutral state overlap priority had something to do with it, but from testing it just seems unrelated. It would make total sense if the foreground player had frame advantage, and it would be easy to tell who has priority in a match and take advantage of glitch cancels.
