The day John Lennon was killed, students began layering graffiti on this wall as a commemoration and it continues to this day. 

In Soviet-Prague, that was a fairly dangerous thing to do, but there were so many students involved it became an insoluble problem for the police: they couldn't even arrest an appreciable fraction of the participants without substantial negative (and world-wide) publicity, so they turned their backs and ignored what the students were doing. 

This quickly became the place where nearly anything could be written.  Kateřina admits to having added her own contribution, though it has long been covered over by later writers.