Ultimate Weapon, if mistake's done only once and it's not used repeatedly or the application is not full of [small/big] mistakes, no senses and however, application is really good overall then they might look through fingers about this small little mistake and accept you. Also mistakes are human, we all can make them - but using "u" instead of you or using entirely wrong grammar is definetly not something what'd make you accepted. But more you read through your application, smaller is the possibility that there's any mistake left - that's why I always said, read it through few times, take 1 day for just reading the application and believe me, it can become much better as it was from the start.
Most of the time people find some new statements they can add to their application, too or find out that they've read the question wrongly. So take your time, don't rush, and you will be fine! (: