Boss Room Invasion in Elden Ring
Out of 100 attempts to invade in the corridor to Radagon's arena, once or twice you might get to invade the host while they're actually fighting the boss. If timed correctly, you won't get the usual "host is fighting boss, returning to own world" message, as you'll have joined the session before Radagon's model has been loaded on your end.
From Soft's probably going to fix this at some point, but until then, enjoy having the Old Monk experience in Elden Ring!
Recorded on PC (patch 1.04.2).