If this happens infrequently, then that's ok, but if you have to go to a reference to be able to follow every 3rd line of code then your productivity plummets compared to someone who has the rules "burned in".

I was interviewing a guy once, and he was having trouble with a simple C programming question I had given him. It appears he never used "for" loops. He only used "while" loops (because you can do everything you need to with "while", and "for" is just confusing, what with 3 different expressions in that one statement).