mech fan blade rpm limit?

This CAN happen. I still vaguely remember about a 67? or so goat came into the hobby shop one day, and there was a hole in the hood looked EXACTLY like an axe made it ..........except from the inside out.

Asked the owner........yup........Fan came apart and a blade through the hood.

Sometimes when I check "total" advance/ timing, I barely warm the engine, and disconnect the fan belt before checking timing.