On the windows of the buses of Isle of Man Transport there used to be clear stickers with red letters warning "THE EXECUTIVE WILL PRESS FOR HIGHEST PENALTIES AGAINST OFFENDERS". With the aid of a 10p piece, it was easy to amend this to "THE EXECUTIVE WILL PRESS HIS PENIS AGAINST OFFENDERS". Quite often, it was noted that the executive (whoever he was) had something of a penchant for pressing his penis against fenders, too. Practically every bus on the island carried these modified warnings at some point during the 80s. They don't any more. Pity.

On a similar line, the signs reading "Please mind your head" on our local trains, reminding travellers to watch out for the overhead luggage racks, were easily and often changed to "Fleas in your head".

At a pub I frequented as a sixth former, a colleague and I set to work on the blackboard displaying the sweets of the day.

"Black Currant Cheese Cake" became the ever so wrong "Black Cunt Cheese Cak".

A green felt tip pen can be gainfully employed to change the logo on a Starbucks Coffee cup to read 'Fuck off'.

That'll show 'em.

In Secondary school we had little 'planners' to write our homework assignments in, and on the front they had the name of the school, Penglais School.
Obviously with Tippex you could easily erase the 'G', the 'L' and the 'A', to make 'Penis School'. One gayer even amended his to say 'Penis Cool', which predictably earned him a severe beating.

A list of rules on a nearby public Pool Area (including one about 'trespass') was unwisely constructed with those little stick-on letters that are pleasantly easy to peel off. This allowed the creation of the line, "NO ASS IN THE POO AREA". Sadly, a new sign was eventually put up, minus the stick-on letters.

Also nearby was a sign in front of an ice-cream shop advertising 'Buttercream Milkshakes'. The 'er' was stolen from that sign about four times within the space of one day before the store just got rid of it. I had to admire their persistence.

Greengrocers used to use easily wiped off white stuff on their windows? TOMATOES could be partially redacted to TITS (the price of 50p a pound seemed reasonable, and was left alone).

If you're in a hurry, however, GRAPES - £2 can be converted in a single swipe. Although it's kind of defeating one of the key benefits if you end up paying two quid.

All the 'Please mind your head' notices on Merseyrail trains were (and still are) altered to the breathtakingly witty 'Fleas in your head'. No wonder all the people who travel on them look like such spackers.

Similarily on the metropolitan line of the underground was the warning "blocking the doors causes delay & can be dangerous" simply remove ing from 1st word, s from the 4th and then "can" and you have a challenge to all like minded people. This has now been changed since re-furbing the carriages and so adequate syntax can no longer be achieved upon scribbling out...

Also, 'please give up this seat if an elderly or handicapped person needs it' on the Underground often became 'please eat an elderly or handicapped person'.

The most creative of these I've seen is an adaptation of 'Please give up this seat if an elderly or disabled person needs it'. Crafty application of masking tape has rendered it 'Please eat a disabled person'.

The secondary school I attended has the dubious pleasure of being located on a road called Bents Crescent, just off Bents Drive. While being open to the obvious merry japery anyway, it took a surprising length of time before some bored kid with an imagination and a bottle of Tipp-Ex carefully blanked out the appropriate letter parts.

A local sign across a footbridge bearing the order "CYCLISTS DISMOUNT" was, through the aid of some judicious blocking in and the addition of a final S, changed to "CYCLISTS IS CUNTS". Grammarly challenged but effective nonetheless.