Measuring pieces with two different rulers and finding out they are not the same.
Mixing precut strips with home-cut strips and finding out the pre-cuts were oversized so now the weaver pattern is off.
Doing ANYTHING in a hurry or last minute. IT (whatever IT is) is bound to end up in a disaster.
Getting to the end of the binding and cutting the excess on the wrong side of the cutting line, making the whole thing 3" too short.
My favorite: Cutting that last bit with finished pieces ON the cutting board. Yep, Cut a wedge right out of every layer and not the one where an applique can come to the rescue.
On the "up" side, thank Hanna for color catchers. They just saved my bacon on a patriotic almost-disaster.
AND, when the chips are down, and the damage is done - there is always that creative spark to make an even better project.