Misinterpretation of the spelling of clumsy. Commonly mistaken by those who are indeed clumsy themselves.
A general indication that a solid grasp of the English language is lacked. If symptoms persist, use a dictionary.
"Don't pick on my grammar! I'm not normally this 'clumbsy'." -irony.
"Yeah that guy's an idiot; he's always so 'clumbsy'!" -irony, once again.