100 Mhz clock frequency for a 16 Mhz MCU
It won't meet timing for one.   All the paths through the circuitry of the chip are designed to run without errors at a certain speed.  Everything is timed to work together it's part of designing a chip.  If you go above that speed then you'll violate those timings and start to hit errors.  

For instance if you had plenty of time for a signal to go high at 16mhz before the next clock, you have a lot less time when your next clock is coming at 100Mhz.

Another thing to think about is heat.  Your package is designed with certain thermals in mind.  As your increased clock rate pushes it last this power rate you risk damage as well.

That's a broad way of stating it anyway.