Just submitted the following feedback to Apple.

Using the quick lookup feature (three finger tap, or "Lookup" from the contextual menu) in applications that predate Lion results in dictionary popups that cannot be dismissed. The word gains the yellow highlight, the popup appears. Clicking anywhere in the field causes the word to lose highlight, but the popup remains. The only way to dismiss the popup is to close the window.

Certain words appear to be more prone to this than others. I can reproduce it 100% of the time with the spelling "cigaret", or even the word "hello".

Apps I've tried this in and reproduced it: Linkinus, Adium, Transmit, Chocolat, Unison, Twitter

I am unable to reproduce it in any Apple applications (I tried Mail, Stickies, Finder, Safari, Address Book)