He dispensed himself from Mods

In 1414 the Emperor Sigismund convened an Ecumenical Council at Constance, with Pope John XXIII presiding. This of course was the anti-Pope. Baldassare Cossa not Angelo Roncalli who convened the Second Vatican Council. Constance is famous for the resolution it brought to the problem of three men simultaneously claiming to be Pope. (Until 2013 it was the most recent example of a Pope resigning). However Sigismund also sought condemnation of the teaching of Jan Hus. He asked the Fathers of the Council date operam ut illa nefanda schisma eradicetur – "take care that that unspeakable schism be uprooted". He assumed from its termination that schisma is feminine. In fact it is neuter since it is a Greek word belonging to the Greek third declension. It goes like σῶμα body. One of the Cardinals pointed this out and Sigismund replied:

Ego sum Rex Romanus et super grammaticam.

"I am the King of the Romans and above grammar." Thomas Carlyle tells the story in his History of Friedrich II of Prussia vol. 1, book ii, ch. xiv, p.153 (Boston, Mass.: Dana Estes & Charles E. Lauriat, 1884). He cites Wolfgang Menzel's Geschichte der Deutschen which can be found on Google books here (scroll to p.477, footnote) and in translation here. Menzel does not provide a source.

From Carlyle it appears that this speech was delivered on 25th December 1414, which means this Christmas will be the 600th anniversary.