I have had my weight belt slip off twice in the past year. The first time was when going down a line in a ripping current, and the line rubbed on my belt buckle, loosening it. As I let go of the line to do a free descent the last 15 feet, I felt the weight had shifted. Grabbed my belt and it almost fell off!namabiru:Losing... the weight system. Sorry, how could you almost lose your weights? Yeah, I know, they magically become unbuckled--the buckle was loose and managed to come off while you were focused on watching the local friendly shark eat your instructor/divemaster. But...
IF you've done your pre-dive safety check, the one's that taught but some people neglect, you should have had the belt secure. Yes?
The second time was when I was extremely overweighted (suggested I wear more weight, as I wearing a heavier rental wetsuit in a new dive scenerio.), I was coming up the line after my safety stop and the line again dislodged the buckle. I was compensating for the large amount of lead with an inflated wing. When the belt came loose, I had to quickly figure out how to hold onto the weight belt AND keep from poppping to the surface. My buddy figured out what was happening, and grabbed my weight belt so I could deflate my wing.
So, yes, your weight belt can come off. Learning how to put it back on is a necessary skill.