How to handle impossible descents (STARs)?

I did notice that I once went faster then -5000 fpm and then the co-pilot told me to watch the pitch.

Take note of the FO's wording:

"Watch the pitch."

He did not say "watch the vertical speed." Huge difference. Your nose can be up while you descend.

This is a Concorde on approach (and is actually descending):

(See also: Space Shuttle Re-entry)

Since aiming the nose down will of course increase your vertical speed in the negative direction, it was coincidental that you were going -5000 at the time you were chastised for your pitch.