As far as language is concerned, what is a mistake, anyway?
There's more than one reason why the phrase We don't need no education probably wouldn't go down very well with an English teacher, but it clearly isn't a mistake. It's quite deliberate - and, again, for more than one reason.
On the other hand...
Here is the beginning of an announcement by a West Yorkshire Police spokesperson after a (very rare in Britain) incident in which a man was shot by police.
I repeat, this was announced by a spokesperson. Someone whose job is speaking.
'During a pre-planned policing operation near to the M62 in Huddersfield...'
Now, is that pre-planned a mistake? The pre- is clearly unnecessary (what would a post-planned operation look like?) but I rather doubt it's a mistake. I think the spokesperson is following a convention that started off as an attempt to make something simple look a tiny bit more official and clever.
But look: sometimes, just sometimes, accuracy really does matter, and this is a case in point. Pre-planned is ridiculous, and it's vitally important we have confidence in every single word of an announcement like this.
Especially one from someone who's paid to speak.
Word To Use Today: plan. This word comes from French from the Latin plānus, which means flat.