The Brief: A 'stream sniper' is someone who attempts to infiltrate a livestream.

DEEP DIVE
Stream sniping can take place in IRL streams and in video game streams. The term stream sniper refers to someone who purposefully enters a livestream. The most well-known stream sniping incidents take place in widely-viewed streams with well-known streamers.
In video game streams, someone may try to enter the same public virtual game as a streamer. In IRL streams, someone may show up at a public place where someone else is streaming. To many, stream sniping is considered an annoyance and a form of clout leeching, or clout chasing. Sometimes, stream snipers use the information being streamed to get ahead in a game and gain an advantage over the person streaming. They then may beat the person streaming or otherwise troll them.
Popular streamers may take measures to avoid encountering a stream sniper. One common method is to create a lag in livestreams so that their exact in-game or geographic location can’t be discovered while they’re there.
Some multi-player video games have official rules against stream sniping and may punish perpetrators by banning them.