Redstone loops get broken timings.

https://www.youtube.com/watch?v=PmXkJIC0Agc

A redstone loop, consisting of 3 repeaters and two torches, acting as a timing loop, will get locked into a stable state, or get an altered timing.

First seen in 125 multiplayer (very common); never happened in 125 single player.

Still happens – much less frequently – in 142.