From Diablo Wiki
The Soulstone Crash was an issue in mid-2002, and was caused by Blizzard trying to fix a duping exploit. The following comes from the Warnings archives.
October 4, 2002
We posted news of this October 4th, thanks to Nobbie for being the first to report the problem. Blizzard posted about it a few days later, and you can view their post here. Insert whatever comment you like about it being "high priority" and still not-fixed six weeks later.
The problem is that in fixing a new cheat that was using the Soulstone replacement (Cain would give you infinite Soulstones) as a way to dupe other items, Blizzard made it impossible to get even one Soulstone from Cain. If you do, the game will crash and you'll be booted from Battle.net, and find it impossible to rejoin for about 30 minutes. Which is a good cheater punishment, but rather annoying for legit players.
Do not click on Cain in Act Four if you haven't done the Hellforge quest and don't have a Soulstone already! You must pick up the Soulstone after Mephisto drops them in Act Three, and hold onto it. If you do that, all is well. Otherwise you'll have to avoid clicking on Cain at any time in Act Four. Since this quest on Nightmare and Hell is one of the best ways to get a mid-high level Rune, you want to save your Soulstone and use it.
Blizzard is aware of this (obviously) and is working on a fix, but as of November 22, 2002, this bug remains.