Lanefan
Victoria Rules
Round here parties split and-or individual PCs go off on their own all the time, usually in situations where they feel (rightly or wrongly) there's little known or perceived danger but much exploring to be done, and simple logic says multiple groups/people can cover more ground than one combined group. The result often resembes the herding of cats... 
Also, any character with flight is going to fly off and scout All. The. Time. And usually invisible, thus the party on the ground can't see which way the scout goes and-or keep track of it.
And then there's unintentional splits. In the course of one adventure it happened twice in fairly rapid succession:
First, due to a wild magic surge each of the ten characters in the party got teleported to a random place in the (very big!) dungeon. Fortunately, they'd mostly already cleared it out, but trying to run each individual character as he-she tried to find anyone else and-or figure out where they were - yeah, that was fun.
Then not long after they'd reunited this happened: during what was supposed to be a rest day the Ranger left to escort a rescuee back to safety. The MU and another PC teleported back to town for some reason. And then the remaining six decided to do some exploring instead of resting, except the Dwarf was left to guard (and count, and recount!) the hoard of g.p. they'd found. The explorers found a teleport trap on the other end of which was a very nasty encounter. The Thief was first in, dead in two rounds. When Thief didn't return the rest went in as a bloc but then got scattered; one ended up falling into a pit trap and just staying there to hide, another fled, and the rest used a device to bail out for home! (so at this point they're now split in six)
Eventually they again got back together (one of the characters having a device of scrying helped immensely!), and carried on.

Also, any character with flight is going to fly off and scout All. The. Time. And usually invisible, thus the party on the ground can't see which way the scout goes and-or keep track of it.
And then there's unintentional splits. In the course of one adventure it happened twice in fairly rapid succession:
First, due to a wild magic surge each of the ten characters in the party got teleported to a random place in the (very big!) dungeon. Fortunately, they'd mostly already cleared it out, but trying to run each individual character as he-she tried to find anyone else and-or figure out where they were - yeah, that was fun.

Then not long after they'd reunited this happened: during what was supposed to be a rest day the Ranger left to escort a rescuee back to safety. The MU and another PC teleported back to town for some reason. And then the remaining six decided to do some exploring instead of resting, except the Dwarf was left to guard (and count, and recount!) the hoard of g.p. they'd found. The explorers found a teleport trap on the other end of which was a very nasty encounter. The Thief was first in, dead in two rounds. When Thief didn't return the rest went in as a bloc but then got scattered; one ended up falling into a pit trap and just staying there to hide, another fled, and the rest used a device to bail out for home! (so at this point they're now split in six)
Eventually they again got back together (one of the characters having a device of scrying helped immensely!), and carried on.