In a video made by Eldimarix, the bug is demonstrated. If Neeko transforms into Kled to disguise herself and is then hit by an enemy Lulu's Polymorph and basic attack, she becomes immune to all damage and the game still sees her as Kled.
This glitch can be abused to enable Neeko to topple enemy towers without minions, dominate jungle mobs, take down dragons and even attempt to slay Baron Nashor single-handedly for her team while being entirely unkillable. It also means duos can queue and try secure Kled and Neeko picks on their team to pull off the glitch in Ranked games.
The Play-In for the 2019 Season World Championship starting tomorrow, but this discovery is unlikely to impact upon those games. Previous Worlds tourneys have seen games paused for hardware glitches and a game-breaking bug such as immortality would not be allowed at League of Legends' most important annual event.
However, regular players may well encounter the bug in their own games. The only solution is to ban Kled, Lulu or Neeko in matchmaking games at the moment. Some have claimed to see those abusing the bug have already faced bans which means Riot are most likely aware of the issue and a fix should be imminent.