Category:No title text
So far only two of the "real" numbered comics have missed a title text. The fact that both 1506: xkcloud and 1525: Emojic 8 Ball do not have a title text is likely due to the fact that they are a "new" type of interactive comic where title text is not an option.
The comic 404: Not Found, although considered a real legitimate comic, though avant-garde, by Randall, it is not everyone who consider it as a true comic. But it is explained here and since it is just the standard error page 404 - Not Found there is of course no title text. As it was "released" on Tuesday April the 1st 2008 (between the normal Monday/Wednesday comics) it is considered to be the first April fools' joke of xkcd.