I love Durham University’s 404 error page. It basically points out all the silly things that you might have done wrong because they couldn’t have possibly left a broken link anywhere. But it’s so detailed: is there any need for this?
“The case of any letters must be correct. Have you got a small letter where a capital letter is needed, or vice versa? Often the l (letter ell) is confused with 1 (the digit 1), and an O (capital letter oh) is confused with 0 (the digit zero). If this may be the case, try using an l (letter ell) instead of a 1 (digit one) or vice-versa. Or, try using a 0 (digit zero) instead of a O (capital letter oh) or vice-versa.”