Grammatically correct graffiti

How many times have you sat on a pub toilet and read all the scrawlings on the back of the door? And how much does it amuse you to, not only see a spelling mistake, but to see someone has come along and corrected it. I am a bit of a spelling and grammar nazi myself, and I can't help but correct people when they say 'should of' instead of 'should have', or when they pronounce schedule like 'SKEDule' (we Brits say SHEDule).

Anyway, I was walking to work today in Richmond and I passed the newly constructed Pret. It's still all boarded up and on the front I spotted this:

This blatant error sparked a two-person correction! The person with the pencil violently circled it and wrote 'Plural!!', and the person with the pen was clearly not impressed and gave it a shameful -1 out of 10 and slapped Pret with an F-. Poor Pret wouldn't even get into sixth form with those grades. You'd think, with the sign being so large, that the mistake would have been obvious to spot, considering all the various people it would have exchanged hands with in the process? 

Anyway, made me laugh, despite the fact our country is so uneducated about our own language.