The MARC->MODS mapping takes 008/34 BK (code a) to "biography" rather
than "autobiography".  (MODS->MARC takes "autobiography" to 008/34 BK
code a, however.)

"game" is mapped to 008/26 CF (code g) only, but it also corresponds to
008/33 MP (code n) and 008/33 VM (code g).

No mapping is given for "multivolume monograph."  How does it differ
from "series"?

No mapping is given for "online system or service," but it should be
008/26 CF (code j).

There is no term in marcgt for 008/24 BK (code u) and 008/24 SE (code
u), nor for 008/21 SE (code #).