Board Thread:ROBLOX help and discussion/@comment-24531239-20170708014007/@comment-26397550-20170708154948

There was a time when you could do some conversions with external programs to a mesh type ROBLOX supported (but never specified that they did) and trick ROBLOX Studio into that it was a base mesh and upload it anyway. This was patched rather quickly but was still in the game for quite a while resulting in those types of meshes