When I want to go look at a specific tag in my "interesting tags", its very easy for me to "misclick" and hit the delete "X" on accident. Its very annoying because I have no feedback as to what I deleted. I have to look over the tags and try and guess what's not there. I feel kind of dumb complaining about it, but alas, it has become rather annoying.
It would be nice if a little info bar linky thingy (highly technical term) under the interesting tags told you "You deleted php click to undo". This is synonomous with what happens when you delete in gmail. You can easily hit "undo" if you deleted unintentionally, and it avoids a stupid confirmation window "Do you REALLLY TRULY want to delete php".
I'll add that although it seems trivial, its the most annoying usability problems I have with the site. I bookmark a good number of "interesting tags", and then I can very easily mistakenly lose one, and I can't figure out what's not there without spending at least a few minutes scanning my interesting tags. This is a significant distraction from my usage of the site.