I am not sure what you mean by fails. Do you mean, get a pet that you dreamed about but had a small chance of getting?
If so, none....I have done one and got a very interesting hybrid.
I have had a lot of failures in dropped pets getting 'stat boosts' instead of powers. After analysis, stat boosts aren't worth the slot...and they are rated at one or two dots. Very good stat boosts may give a 1% add to a power...not enough to make it viable to advance the pet further.