What Lens do you most regret selling?

I don't know if this is just me but if I don't really use a lens that much I generally sell it to finance a lens that I could put to more use.

The downside of this is that every now and again you think to yourself, 'I wish I had that old XXX for this shot' but of course, I have sold it.

The lens that I most miss (and I only had it a very short time) is the 10.5mm Fisheye.

How about you?