- Warning: this comic occasionally contains strong language (which may be unsuitable for children), unusual humor (which may be unsuitable for adults), and advanced mathematics (which may be unsuitable for liberal-arts majors).
Four hours later that day the page looked like this with no footnote or warning.
When a new footnote appeared this warning did not reappear.
The warning thus lasted almost 10 and a half years. Many xkcd fans will likely be sad to see that it has disappeared.
See Design of xkcd.com.