In preparation for a puzzle geocache that I've been working on, I was investigating some of the lesser known properties of ellipses and this naturally led me to Eric Weisstein's Mathworld website. I found an error and wrote the Mathworld people about it and I got a personal response from Eric. It felt like a nerd celebrity encounter or something. :)
The issue at hand was the statement that "Four nonconcyclic points uniquely determine an ellipse..." This is clearly not true (consider the verticies of a triangle and any point inside the triangle). The correct statement is either that "Four nonconcyclic points on an ellipse uniquely determine the ellipse," or "Four nonconcyclic points that represent the verticies of a convex polygon uniquely determine an ellipse."
Anyway, my correction is going live with the next update (no idea how often that thing gets rebuilt). So I am now a Mathworld contributor. Cool!
I hereby dub you Mouser, King of the Nerds.
I once found a very obscure bug in nethack---an accomplishment which immediately reveals how much time I spent playing nethack at one point in my life. Certainly the process of filing a bug report is nothing exciting, but it was nice to get a response back from someone on the actual nethack dev team a couple days later: "Thanks. Fixed in next version." But I didn't get an email from Jay Fenlason or anything.