A t-shirt available from the Mental Floss store:
Copy on the site:
This is the grammar police. You have the right to remain silent. Anything you say can and will be held against you in a court of grammatical law.
Grammar Police is an instance of the X police snowclonelet (which I haven’t posted on before), and a very popular one at that. But if you look at some of the enormous number of sites using the expression, you’ll see that most of them aren’t about what linguists think of as grammar, but about what I’ve called garmmra (largely spelling and punctuation).
From the Morning Glory site (which has a Grammar Police category), a police badge:
and a graph illustrating the sorts of things that the Grammar Police try to regulate:
In a similar vein, there’s a YouTube video about their / they’re / there:
Smosh: Grammar Police
You can run but you can’t hide as Smosh takes to the streets to regulate bad grammar
and the Grammar Police family of web sites:
Grammar Police a.k.a. GrammarCops
Gr8 Grammar (NBD, J2LYK, this blog is our 02 on txt abbr n product names 4 u 2 C n LOL 4ever!)
and of course the news story about two guys who got into trouble by “correcting” signs in U.S. national parks (among other places):
Grammar police punished for ‘fixing’ rare sign
Self-styled vigilantes wiped out errors on signs across the United States
On Language Log, grammar police has come up a number of times, in some cases in association with other instances of X police:
AZ, 6/22/05: Enforcer Syndrome (pre-adolescent phase) (link)
“Douglas has gotten into grammar,” explains Jane [his mother]. “He’s an officer in something called the grammar police.”
“Word police,” corrects Douglas.
AZ, 9/5/05: Cartoonists on the grammatical front line (link)
[quoted] “Jump Start” always has correct language–and a character who is a member of the Grammar Police
ML, 9/21/05: You da man (link)
[in BC cartoon] Oh, great, here come the grammar police.
GP, 10/3/10: You can get preposition stranding right to start with (link)
[quoted] in the announcement of the invitation for us to become grammar police, I found two errors …
BZ, 7/2/11: The idiom police, if you will (link)
[in Candorville cartoon] A: Who died and made you the grammar police?
B: Idiom police.
Note word police and idiom police. Elsewhere you can find comma police, apostrophe police, quotation police, punctuation police, and spelling police. And outside of garmmra, there’s sex police, liquor police, morals police, fashion police, and many more (sometimes subsective, sometimes resembloid, as in grammar police).