While all of the Doom 2 maps render as intended, several Doom 1 maps have so called "unclosed sectors" like in
Sector 7 (the large room just below the normal exit) is not "closed", because linedef 927 originates along the body of linedef 933 rather than at its endpoint. As the sector is not tagged, however, this error has no observable effect during normal play. -- Doomwiki
Sector 1 is unclosed, as the front sidedef of linedef 21 and the back sidedef of linedef 57 reference their own sector (sector 2) instead of sector 1. -- Doomwiki
While all of the Doom 2 maps render as intended, several Doom 1 maps have so called "unclosed sectors" like in