After some random recent upgrade, emacs started doing something annoying: when in TeX mode (eg when editing LaTeX docs, which I do a lot), it would treat every underscore in the document as a signal that the next symbol should be subscripted, and display it as such: the character would be offset vertically using font-lock magic. This is very annoying, because underscore only means subscript in maths mode; the rest of the time, it’s wrong and silly to visibly subscript the thing. In general, I don’t want emacs to do such semi-WYSIWYG-ness – syntax highlighting is about as much as I need, thank you.
Anyway, how to turn it off is non-obvious, and required several increasingly imaginative google search strings. In the end I came across this recent thread on gnu.emacs.help, one of whose posts suggested that turning down font-lock-maximum-decoration from “maximum” is the right thing to do. Yep, that works: I changed it to “nil” and now I’m happy.