Apollo 1. Three astronauts died.

https://en.wikipedia.org/wiki/Apollo_1

Not to mention there being a lot of launch failures pre and during the Apollo era, including pad explosions (there are nice compilations on youtube). But that was not really that much of an issue, as this was expected and there were dozens of pads built for these launches, so the testing cadence was not affected.

There was no fatal launch failure for Apollo & pad explosion would be a problem with just 2 pads available.

There were a couple Saturn V stage explosions during testing but again - those damaged test stands, not the pad.