In frame.el's toggle-frame-fullscreen and toggle-frame-maximized, these
look like either typos, or vestigial references.  The frame-parameter
fullscreen is never set to 'fullscreen nor is the value 'fullscreen used in
other places in Emacs.  It is an undocumented value, appearing nowhere.
Unless I missed something.

I reviewed below sources and relevant headers:

android.c
androidfns.c
androidterm.c
emacs.c
frame.c
gtkutil.c
haikufns.c
haikuterm.c
pgtkfns.c
pgtkterm.c
term.c
w32fns.c
w32term.c
xfns.c
xmenu.c
xterm.c
nsfns.m
nsterm.m

-Stephane