These are just my personal observations, but hopefully the info will help someone:
I have found that all of the times that I stumble or fall have a single common factor.
I was doing two things at once.
And these don't have to be two physical tasks. Anything that requires thought.
Walking? OK, so long as I concentrate 100% on walking.
Looking at something? OK, so long as I stop walking before looking.
Walking and looking at the same time? Erm, no. Body meets pavement time.