Is Your Harry Potter OC a Mary Sue?
Oddly colored hair (naturally or dyed) ie: blue, pink, streaked hair, "bottle red" (Think Rihanna)
Oddly colored eyes (and colored contacts) ie: red, yellow, pink, COLOR-CHANGING EYES
Unnaturally beautiful characters, ie: perfect proportions and complexions, etc.
Being an animagus or a metamorphous (meta is very, very rare and ani is hard to achieve and you have to be registered)
Being a half-breed creature, ie: half-veela, half-vampire, half-werewolf (or a werewolf, for that matter)
Having your character self-harm, ie: cutting, popping pills, etc. (This is a very over-done thing in oc's, and it's frankly not something healthy)
Having them dress in clothing outside the time-period (do your research)*
Having them be goth/emo/scene (seriously over-done)
Giving them tattoo's and piercing (they go to a boarding school in Scotland, seriously guys)
As just a general statement; "Scene", and "Emo" fashions didn't start up until the mid-to-late 2000'