On the week of November 8th, 2010, XKCD's creator put out a comic saying that he would not be drawing new comics for a little while because of family illness. In the Alt-Text, however, he asked for people to send him distracting online games. The result? 2700 by noon of the same day.
He revealed exactly what the family illness is in July 2011: his fiancee has Stage III breast cancer. In that blog post, he thanked all the people who sent him flash games.
This strip may not seem too special, but then you read the Alt-Text...