General :: Slackware - Ubuntu Machine /home/_ Folder?

I've got a machine running Slackware64 13.1, and Ubuntu 10.10 32-bit, and today, while installing python-ogre in Ubuntu, I was informed that I had run out of hard drive space on my /home partition.

The /home partition is shared between Slack and Ubuntu, and just earlier today, upon booting into Ubuntu, I got a message that the /home partition couldn't be mounted (before most of the system had loaded. It was still on the purple loading screen) Well simply rebooting fixed this. Then today I suddenly ran out of space, and I thought I was only about half way through the allotted space. Well after some investigation I discovered a folder called /home/_ which was an exact duplicate of /home. Well in need of some extra hard drive space, and in conclusion that it was unneeded, I deleted it. And when I deleted it, everything in /home deleted as well.

First off, what is this /home/_ folder? Where could it have come from? The creation time is about 7 minutes after I created a new user on Slackware, and I think it might have been about 7 minutes after creating that user that I had changed its home directory so that Ubuntu's user config files wouldn't conflict with Slackware's.

Secondly, why did /home's contents delete when I removed /home/_? I already figured out there isn't any way of recovering those files (except for the files I luckily synced with Dropbox)

Third, is there some way to prevent this from happening again? Does anyone have any experience with this? Has anyone heard of this happening before? I already know that /home/_ was not a symbolic link.