Even at the processor level every single transistor on it has a rated mean time between failures a.k.a. MTBF. Sure it may be astronomical, but you do have a lot of transistors, so in practice a random bitflip is not such a rare event. Designers actually explore MTBF vs power usage trade-offs here, and there is even a fascinating area of "fault resilient computing" research.

Every single clock domain crossing has another MTBF (google metastability). Again they are very high (billions of years if done properly), but you will have plenty of such crossings (and the number keeps growing with modern, more asynchronous design).

Processors are quite unreliable things.