Speeding up the processor results in speeding up the internal system clock, it's the heartbeat of the system and measures almost everything.

If you're speeding up too much / heating up the hardware to limits the clock dependant components will miss a beat or see it as "background noise" (remember, everything is analogue signals, they're being digitized and filtered).

So the clock will have somewhat an arrhythmia and therefore precise timing operations are more likely to fail. This includes RAM and also disk- / mmc-writes.

The faster you go, the faster will the flanks of the signal rise and fall.
The lower voltage you go, there is more possibility the signal will decarded as noise.

Explained in layman's terms, reality is somewhat complexer.

Good news:
- it COULD break your filesystem -> reflash
- nothing to worry about if you keep regular backups (!!)