When replacing text with the :s command, if there is a special escaped character (such as "\t" for tab) in the replacement pattern, then fakevim actually puts "t" (without the ).

This only affects special chars in the replacement pattern, not in the search pattern, so for example this one works as expected:
:.s/\t/X/g
(replace all tabs by X – note that searching a pattern such as /\t also works correctly)

But this one doesn't work:
:.s/X/\t/g
(should replace all X by tabs, but actually replaces all X by t)

This cannot be worked around by escaping the \ char as this puts a literal \ in the result, as expected:
:.s/X/
t/g
(replaces all X by the literal characters "\t")