I came across this while looking for something else.
It is put on the backs of buses in London that are in use for training new bus drivers.
The OP wrote "It's incredibly disarming. It would be good if someone could attach this message to the internet and then maybe everyone would be nicer" - but that applies to much more than the internet. It is also amazing how often people needed to be reminded that mistakes are a necessary part of the learning process.