I get it "all the time", whatever that really means.
I had thought I'd narrowed it down to whenever the rar filename contained non-alphanumerics or blanks, but I can't discount DDT2005's explanation either. I have altbinz open most of the time in the systray, which means that the chances of attempting to import a rar'ed NZB file after having previously unrared an archive are pretty high for me.
I might try running it under a debugger and see what the backtrace is saying about where the error is coming from, i.e altbinz itself or a DLL it uses. It's got to be an unchecked (ahem, naughty) pointer returning NULL though, doesn't it?