One of my shelf making experiences had the dado on one side off by ~3/4". Since it was pine and a long shelf I just muscled it in to place.
I have since taken to putting the sides back to back before marking the dados and checking a few times during progress. Most alignment errors now are small from a saw being on the wrong side of a line and easily corrected.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)