I like three very much:
1) "we are dealing with a sysadmin"
2) Math over physics over chemistry over biology...
3) Knuth's answer to array index inconsistency across languages (some start from zero others from one because [reasons])
1) "we are dealing with a sysadmin" - http://xkcd.com/705/
2) Math over physics over chemistry over biology... - https://xkcd.com/435/
3) Knuth's answer to array index inconsistency across languages (some start from zero others from one because [reasons]) - https://xkcd.com/163/