So far, the worst mistake I've made was getting into a hurry when doing a BIAB batch and pouring about 1 1/2 lbs of grain into my brew pot without the bag in place. Needless to say a mess in straining it out, but the beer turned out fine.
At the time I was new to the BIAB process and was afraid of burning the bag as I heated the water to strike temp.Now I use an inverted S/S vegetable strainer at the bottom of the pot to keep the bag off the bottom of the pot and put the bag in place before filling the pot with water.
I've since made a check list customized to my equipment and that really helps.
Regarding drinking while brewing, I have no more than two while in process. It does make the time more enjoyable.