Passing "blob:" as the URL to showModalDialog() opened an invisible modal window — one that was present but not rendered — making the entire browser unusable until the process was killed.

showModalDialog("blob:"); // Invisible modal renders the browser unusable.

The modal window opened but nothing was visible to the user, so there was no way to close it. Because showModalDialog blocks the calling page, the entire tab became unresponsive.

Found during my years at Microsoft (2006–2014). These bugs were patched long ago — shared here as a historical record for learning purposes.