As the testing of the F1 evolved, it became clear that harmonic instabilities were causing failures. Of course, these instabilities had always been present in rocket engines. They are caused by acoustic pressures building up unevenly and zinging around inside the engine. In the smaller combustion chambers of engines past, they were not so destructive. But inside the massive F1 combustion chambers, these instabilities could become crippling.
Tired of watching their rocket motors exploding, Rocketdyne engineers decided to take control of the process in a unique way. Rather than allowing the acoustic instabilities to initiate and build on their own, they took a "sour grapes" approach, designing an explosive device that could be inserted into the engine upon firing on the test stand. Then, rather than waiting for the instability to build up unpredicted, they could set off their bomb (which was of a known power and yield) and watch a similar acoustic instability build... but with increasingly predictable results.
Eventually, between tests such as the above and many, many redesigns of the injector plate among other components, most of the problems were resolved and the F1 engine was tested in flight and declared flightworthy and man-safe.
However, this is not quite the end of the tale... for as the Apollo program sped on to it's triumphant arrival on the moon in 1969, the Saturn V and the F1 engine continued to have teething pains... the Pogo effect.