Definitely exist. Example would be the Prime X470-PRO's M.2_2 socket. The reason they do this is to assist PCIe 3.0 x2 drives which are limited to 2 lanes even if the M.2 socket is 4x PCIe 2.0. There's a lot of these: WD SN500, any of the E8/E8T drives, SX6000NP, etc. In that board's case the socket shares a PCIe clock with one PCIe x1 slot and bandwidth with two PCIe slots (these slots share bandwidth with the x4 electrical x16 physical PCIe slot from PCH). I'd say this was an error but I've seen multiple boards as such, like the TUF X470 Plus Gaming, although some of their boards allow x4 PCIe 3.0 by pulling lanes from GPU.