I usually only dropped off into the abyss when I was loading a server and traveling too fast over terrain that I'd not mapped out yet. Although one time I did cause my base to collapse as I flew in from creative mode and was in the area of my base and the pieces of the base loaded in a funky order and I guess caused an SI issue, then poof the base fell apart.
I've hit the abyss a few times also when digging, which is yet another reason to always carry frames with you (dogs and bears being the most important reason!), but anyway, when digging if you hit a spot that looks diggable, but your shovel keeps hitting empty air, try putting some frames up around it. Due to the nature of the game, it should straighten the terrain out and make it less likely to fall through the landscape.
Of course sometimes that does not work which is why I usually have creative running in the background, and I'll use it for just that purpose and fly out of the hole, land nearby, and then see if I can fix the issue with frames.
