The following 12-character input causes TeX to go into a loop.
\def\x{\x}\x
It won't crash, it won't produce an error message, it'll just sit there for ever, trying to compile the document (but not getting anywhere because no matter how many times it expands \x, it just gets \x back again).
Is there a shorter input that confounds TeX in a similar way?
\def~{~}~? I am withholding my vote, with an inclination for close vote. – Masroor Dec 29 '13 at 16:10