After editing a wiki page a security code (captcha) has to be entered for confirmation and to prove that a human made the changes. The code (mixed digits and characters) is displayed in uppercase letters, but entering the code exactly as shown always leads to an "incorrect security code" message. Apparently the letters have to be given in lowercase form.
Additionally for people not immediately aware that only hex digits (0-9, A-F) are used here, the zero (0) might be confused with an uppercase "O". A short hint about the valid character range might be useful.

Captcha for wiki page edits confused by uppercase letters