I've noticed recently, while reading a lot of older comments, that the TeX seems to be broken; for example, a command will appear like '\rbra ce', and so MathJaX complains that it doesn't know the command '\rbra'. I thought that coming across so many instances of this odd error was, well, odd, but just wrote it off to chance.
However, I just noticed that one of my old comments also suffers from this same issue: Replacement for Lie-algebra complements . (Note the sequence '\mathr m'.) I am as much capable of typoes as the next person, if not more so, but I am almost completely positive that my command '\mathrm' did not have a space when originally posted, and that it compiled correctly. Is there any chance that some recent SO upgrade somehow managed to introduce spaces in the middle of TeX commands in many comments?
EDIT: Asaf Karagila points out the MMSE discussion of exactly the same issue, and @Sklivvz points out that it has now been fixed. As @S.Carnahan mentions, this comes from the way the TeX was being parsed, not from the TeX itself, so my linked comment above no longer displays the issue.