In a park in Ealing, London (was it Ealing Common?) we came upon this notice:
The arrival and that of their descendant’s have changed the face of Ealing and England forever.
If you look carefully, someone tried to erase the offensive apostrophe.
Someone caught in the act of adding an apostrophe earned himself the epithet of “punctuation hero”. His neighbours weren’t too pleased though. Read all about it in the Mail Online.
Bridging the Unbridgeable 
In the ladies’ toilets at Oxford station there’s a sign saying that the toilets are cleaned ‘on a regular basis’. Someone has crossed this phrase out and replaced it with ‘regularly’. (I’m afraid I don’t have a photo.)
And here’s a link to a similar corrected sign, this time double negatives: http://stancarey.wordpress.com/2010/03/26/grammar-to-go/
It would be interesting to collect corrections like these and find out which types of usage people are more inclined to correct!
Thank you to Kate for the link to my blog. The photo above is too small for me to see the attempt at erasing the erroneous apostrophe, but the text suffices.
I once blogged about something very similar that I spotted in a comic: a rogue apostrophe that someone tried to remove, in this case more successfully. A mere smudge remains, to which I gave the name apostrophantom.
This post reminded me of “The Great Typo Hunt.” http://greattypohunt.com/
Is the apostrophe in “neighbour’s” the deliberate mistake that no-one else has seen, or is it so obvious that I’m the only one who’s bothered to point it out?