Keg, 2121 19 bytes (1919 17 characters)
abc#( 1abc#1 / ╘ " a b c
Simply prints abc and the rest is a comment.
Fails in MathGolf due to an undefined operator overload of /. / can not work on an integer (1) and a string( pushed via the space instruction.)
MathGolf
ac(1ac1/╘"abc
Does a bunch of nonsense, then ╘ deletes the whole stack. "abc printspushes abc and the stack has an implicit output.
Fails in Keg due to the failure of implicit-outputting a floating-point number in its character form.