Nice little minor-mode that will make your buffer all nice and flashy when you break some formatting rules, that is, it makes great reason to start TabsAndSpacesDiscussion? all over again 
Available at: http://www.jpl.org/ftp/pub/elisp/develock.el.gz or at your near Debian repo (that’s the way I actually learned about it, Debian packagers keep amazing me 
- Does anyone find this useful? It seems that this is just the kind of annoying highlighting that cperl does (and I turn off); I rely on {()} balancing going out of whack to know when I’ve made a syntax mistake. --ErikBourget
- I tried it in Lisp mode and it filled my buffer with so much ketchup and mustard I nearly fell off my chair.
- This mode makes the whitespace pedant in me very, very happy. --MichaelOlson
- I find it helpful. I’m trying to figure out how to activate it for ProofGeneral’s CoqMode --SamuelBronson
- I personally like how it highlights text that goes past column 79, as well as trailing space. To the ketchup and mustard guy: move point to ketchup and/or mustard; M-x customize-face; remove background color or make more subtle; done! --DarrenEmbry
ProgrammingModes