Perhaps there is no WIFI, the computers don't work, the photocopier is broken, there is a power cut, the number of students in the classroom is unexpected, the activity you had prepared for the whole hour finishes in just thirty minutes or the other way around, the activity you had prepared for twenty minutes goes on for the whole hour…and so on.
So yes, we do need a whole alphabet of backup plans for our lessons:
A for adaptation. This is the most important of all. Every activity should be able to be adapted on the go.