There were serious problems with unstable combustion causing the motor to break up, that were fixed by putting baffles on the injector plate to divide up the volume, where the combustion started, into smaller independent regions [1]. Once it was running smoothly, the engineers were still concerned that it might be on the edge of instability, so they ran some tests in which a small explosive charge was detonated in the combustion chamber of a running motor, to check that it settled down to running smoothly afterwards.

[1] http://heroicrelics.org/info/f-1/f-1-injector-baffles.html