Overclocking will damage your CPU in the long run and shorten its life (duh), but turning it back down down’t help anything. The major problem with OCing is that eventually the transistors will start “leaking” electrons, causing data errors. This is caused by deterioration over time of the transistors caused by overvolting.
That said, some CPUs (generally the more OCable ones) are underrated at the factory and so will OC safely to higher speeds. The C300A and P4 1.6 come to mind here. This is mostly a function of the manufacturering process being too good, and mostly happens at big chip companies, e.g. Intel and TI, and less so AMD.