Semper is an unfathomably rare entity, and because of this, little information is known about Semper, other than being a freaky gmod glitch.

There are many theories out there speculating about the existence of Semper, asking about what he is, what he does, and why he does it. He normally appears in either old Gmod servers, or in default maps, and would constantly show up when the player least expects it. Semper is an unknown/unidentified entity that has been the subject of countless sightings within Gmod. So they'll spout lua errors in console and change that model's thing to player mdl.' ― 藤原 拓海 ('Takumi Fujiwara') The server sometimes cant force a model over to error.mdl because of enforced playerstates in some servers. It has happened to me and its the result of missing content. It's a script error when compiling missing model content.The default is to switch to player.mdl which has been a thing for player entities since the 2004 release.

Unrecognized 'The gmod authors never would included stuff like that intentionally.