This seems to only work if the letter you TAB with does not exist in the [chanfolder] section of mirc.ini. I deleted the entire section and #x worked. I then added:
After doing #test, it would not freeze with those letters (a/b/c), but once again froze with #x.
This did not happen with my normal install of mIRC 6.16 which has the default [chanfolder] section in it, even though I tried using a letter that doesn't exist in the default chan folder (#z) nor that I was in at the time. When testing on my normal install, I was connected to 3 networks.
When testing with the 6.16 fresh install, CPU usage did go up to 100%. I also noticed that two instances of the fresh install appeared in the task manager (after it froze up), but I only had one open.
OS: XP Home SP2
Version: mIRC 6.16, completely fresh install.