Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the contents <math>\hat{}</math>. This generates the following MathML: <mover><mo>^</mo></mover>.
"Invalid markup: Incorrect number of children for <mover/> tag." is printed to the Firefox console.
The same problem happens with the macros acute, grave, ddot, tilde, bar, breve, check, hat, vec, dot, widetilde, widehat, overrightarrow, overline, underline, overarc, overbrace, underbrace, overrightarrow, overleftrightarrow, overset{}, underset{} (and possibly more). These macros are usually not used with an empty argument {}, but sometimes you want to talk about the symbols themselves as in "the hat symbol <math>\hat{}</math>" (see the 12 enwiki articles and the 11 dewiki articles where one of the above macros is used with empty argument).
