When I stop predicting; in other words, when I stop reading the pattern or following the gaps due to a lapse in concentration: Sometimes I will accidentally fixate on a relatively small aspect, like a single bullet or gap, and fail to take into account everything else going on around me--usually I instantly realize my error and see how to get around it, but am not left with enough time or space to execute. Other times I'm so into that "zone" that I will mentally note and flag a single bullet as a threat. . .and move right into it, because by then I'm so used to just having my fingers follow what my eyes see.
Sometimes I also over-memorize and commit things to unconscious thought. Like during the X-Multiply week in Akyu's, I'd practice particular parts of stages so many times that on future runs I'd execute the same movements, timing, and tactics that I had previously practiced, while failing to take into account any possible variations due to, among other things, my timing or positioning being off.
Then there's Shou, with her curvy lasers and fireball bullets that lack an outline--no words.