Hi stwert,
Thanks for your reply. No I don't animate with autokey so it could not be that.

Yes the new keys were on a fraction of a frame and were in fact minimally different that the 'real' key (.11 difference).

The scene had like 100+ keys so it is not possible I hit accidently the 's' key on the fraction frames.

Anyway I was able to back track with a previous version thanks to autosave but still, it's disconcerting.

Cheers....