Monk writes:

Here's my crack at answering this question. To operate the engine
above and beyond it's design will result in premature engine wear and
tear. Also at or above the design parameters, the engine is not
getting adequate lubrication and/or cooling, again resulting in
premature wear and tear or possibly even cause engine seizure. This
wear and tear will also shorten the Time Between Overhaul TBO.

Okay, thanks. The last POH I looked at (Bonanza or Baron, not sure which)
said that it was "not recommended," which sounded gentler than "forbidden,"
and implied that it just wasn't a good idea, but wouldn't necessarily cause a
catastrophic failure of the engine.

The POH also seems to imply that operating with full throttle at less than
maximum RPM is okay, and so is operating with maximum RPM but less than full
throttle. I think it also says something about not being at maximum EGT for
too long.

I was mainly worried that exceeding some mysterious limit would cause an
engine failure in flight.