I was trying to do something like this:
Trying several combinations - maybe something doesn't exist. Maybe I've mixed mootools and jQuery too much. But at last I've found a solution and I hope that my post will help somebody.
In the form I had (I've got this form from the Internet) an input of type "submit" which name was (?!?) "submit". Very strange in my opinion - so in fact when I was typing .submit() the browser was treating this input as a function - and that's the reason of this creepy error.
So watch out and try to avoid copying forms from the Internet.