Does anyone have a story of a time where you performed something entirely improvisation. No prep, not knowing what you intended to do.

I suppose that there are some performers who could do that well. It's not normally my "cup of tea" though!

Seems too risky. Unless you know quite a few impromptu effects and gauge an audience for what type of effect they would like

As much as I would like to improvise for some reason in a show, many years experience has taught me to always have a backup effect which I always do.

However Im sure many of us have improvised not a full routine but during a routine you normally do by having something in the wrong pocket or
using a prop not done something you were meant to which happens occasionally in shows.

Once I was doing an Invisible deck routine and picked up the wrong deck to put in view for the audience of about 30 watching, usually I get 3 people to create the card, colour,suit nos etc with the deck in full view already but forgot to do this.., however this time I improvised , had the 1st deck shuffled, passed around etc, 1 card selected that matched the 2nd deck I bought out during the earlier process..