Recently I started writing my MSc dissertation. In doing so, I've been using BiBtEx. Since I'm a computing student, it's not uncommon for me to cite webpages; these need the URL in the reference which often contains the "_" character.

Today, I cited a page with the URL "http://www.facebook.com/note.php?note_id=76191543919" (a good article, by the way, on Facebooks filesystem architecture). Latex came across this and interpreted the _id=76191543919 bit as an equasion, and helpfully tried to add $ into the mix.

Since this isn't an equasion, adding a \ in front of the underscore (so the URL becomes "http://www.facebook.com/note.php?note_id=76191543919") sorted it all out. No more Math URLs!