I modified something in my sidebars and deleted some of my widgets and after that, my sidebars returnes the way there were before i started modifying them in the administrators panel.
i tried over and over againd, and same thing happenned. now, instead of deleting them, i draged them to the Inactive zone, so i can modify the sidebars...
so far, it is the first bug in 2.9.1 - RC1
PS: i have been using WordPress for 4 years and i've been using my current theme since february 2009. it's the first time when i have trouble with my blog functions. i had another error but it was fixed by RC-1.