'Stuff' does happen. However, being a designer and knowing this, I leave myself room. For example, you could have started all your fixtures at address 101 and left the first 100 channels of DMX for additions. I do this with consoles with one universe a lot because last minute things can quickly be added. For bigger consoles, I would get around it by just putting it into another universe or the Hog again just doesn't care.
I rarely use all 4 universes at once on the Hog, and even the it is to save on cabling and not so much that I need 2048 channels of DMX. There have been a few times where I have brought in another DP8000 (processor for the Hog which adds 4 more universes) again just for cable saving. One Cat5 to back of house gets me 4 universes back there and 4 out front then.
I guess it isn't really a designer should know as much as designer experience in this case. When I design, I know what the set is, I spec the fixtures, console, do the patching, spec all the cabling, etc. Within all that however, I leave room for changes and/or additions based on what I am using/doing.