It means you inadvertently enabled “Freehanded Editing”. You can find it in the menu under Edit→Mode, or hit ⌥⌘E to
toggle
it.
This happens a lot to me because Command-Option-E is the keyboard shortcut to clear Safari's cache. As a web developer who codes in TextMate and checks work in Safari (primarily), I'm always hitting Command-Option-E to clear the cache, and sometimes I _think_ I'm in Safari but really I'm in TextMate (really...just because my eyes are looking at Safari, doesn't that mean that it has focus?), and so while I've entered Freehand mode a fair number of times, never once has it been intentional.
I tried to address this by going to the Keyboard System Preference pane, and adjusting the keyboard shortcuts for TextMate...but it looks like it only works for adding new ones or changing existing ones...I don't readily see how one could completely remove a standard shortcut. I did the next best thing and changed it to a really really really obscure shortcut, one I'm not likely to ever press accidentally, but I'd be interested in knowing how to remove pre-existing shortcuts, if anyone knows...
Thanks, Dru