In the UK some pubs still have outside toilets around the back of the main drinking establishment in some old outlying building. They are often freezing cold, full of dead bugs and smell of pungent 'urinal cakes' which are used to disinfect and mask the underlying odour of stale urine.
One day I chanced upon a cigarette having been stubbed out on the window ledge in one of these outside toilets in an old Bristol pub. I visited the same pub about a week later and found, to my surprise, when needing the toilets again, the cigarette butt was still there.
It was at that moment I set about to preserve this simple moment of an unknown man stubbing out his cigarette. So, chipping off a sample of paint from the window ledge off I went to a local DIY shop and, using their paint matching service, purchased a small tin of quick drying gloss paint that came as close as possible to fragment taken from the toilets.
I then returned to the pub, armed with said tin of paint and a couple of paint brushes tucked away in my coat. After a pint or two I returned the toilets, locked myself into the cubicle and quickly painted over the cigarette butt and the surrounding ledge. I repeated this process three more times over the ensuing days.
The fourth time I went the cigarette butt had gone with just a small round mark showing where it had been.