After Valve gave Counter-Strike: Global Offensive map de_train a lick of paint last week, fans thereafter discovered a cheeky exploit had snuck past quality control. The exploit allowed players to surf on pigeons and get a hugely helpful overview of the map and access to advantageous platforms on bombsite A (you can see it in action here).
Since the exploit has been removed Valve has left a memorial for the temporary feature (much as they did after the recent DreamHack controversy, where they left a message that said "no climbing on ledges" upon fixing that exploit). This time the studio left a sign that PCGamer reckons says something along the lines of "on bird not tread". Wise words indeed.