(And a couple for our older listeners...)
Many moons ago
DANGER MOUSE! appeared in 20 foot letters on a railway arch round my way. A few days on and someone else had added underneath, in much smaller letters,
(and Penfold)
But the best 'amended' graffiti I ever saw was (I only ever saw it in its completed version, tho' it had clearly been done by three different people):
ULSTER SAYS NO!
But the man from Del Monte, he says yes!
(And he's a real Orange man)