According to this list http://oxforddictionaries.com/words/common-misspellings it's not a 'common misspelling' but it is in this list http://oxforddictionaries.com/words/commonly-confused-words making it a 'commonly confused word'.
"Common misspellings". Oxford Dictionaries. Oxford University Press, n.d. Web. 04 September 2014. http://www.oxforddictionaries.com/words/common-misspellings.
"Commonly confused words". Oxford Dictionaries. Oxford University Press, n.d. Web. 04 September 2014. http://www.oxforddictionaries.com/words/commonly-confused-words.
Notably, in the original, the author does spell allowed correctly later on in the text, so they do know the correct spelling. I'd go for brain fart.
Brain fart. (2014, August 31). In Wikipedia, The Free Encyclopedia. Retrieved 17:01, September 3, 2014, from http://en.wikipedia.org/w/index.php?title=Brain_fart&oldid=623528981
A brain fart (may be jocularly derived from "brainstorm") is slang
for a special kind of abnormal brain activity which results in human
error while performing a repetitive task, or more generally
denoting a degree of mental laxity or any task-related forgetfulness,
such as forgetting how to hold a fork. ...