Workaround for Caspers [Levels I-IV]
Caspers are avatars who appear to you as clouds of gas. I say TO YOU because they may not be caspered to me. It seems to be largely a client-side problem. This means the problem lies with the software on your computer and not the server at Linden Labs.
Usually caspers resolved into avatars, but not always. Occasionally a cloud of gas will hang around seemingly forever.
There's a workaround when this happens. Go the Edit menu at the top of the screen and select groups. Select a nonactive group and activate it. Then activate the group you just left.
More often than not, casper will be gone.
The above is called a workaround. When the Lindens seem unable or unwilling to fix a problem; some citizen somewhere usually discovers a way to resolve the problem.