The Annual Hat Clusterfuck is the yearly tradition of arguing over the Christmas themed RationalWiki logo. Following the slight re-brand in November 2009 that gave RationalWiki a logo that wasn't copyrighted by someone else, a version featuring snow and a Santa hat was added in December.
Naturally, this leads to a massive argument every year about whether it should be used at all and when it should be taken down. Some like to keep it up for a full festive period going the traditional 12 days after Christmas, others prefer it to disappear as soon as midnight chimes on December 25th and the shops break out the Easter eggs. More extreme opinions involve RW not using it at all as Christmas has - shock horror - religious connotations, but these people are boring and are usually ignored.
In December 2011, much to everyone's surprise, the argument was relatively muted. Nowadays no one cares.