Surprises are good for birthdays, but not useful in traffic. The sooner you know what someone else is doing the easier it is to adjust to it. So scanning behind, having clear and visible hand signals and using the correct lane positioning when turning left, for example, will make you predictable and thus avoid confusing and/or dangerous situations.
I had it happen twice that I had shown a clear hand signal when making a left turn, but was passed by motorists close to the intersection anyway. It happened because I didn't have the correct lane position - I was too far to the right. I don't think it registered with the motorists that I wanted to turn left.
Today I know that riding as far as possible to the right (because that is what is being taught!) is exactly the root cause for this behavior. It doesn't happen to me anymore, but it is a learning proccess, that can be accomplished by lots of practical training.