I just read a great post on Head For Red called bored magi, but which is really about the magicians ability to speak things into being, even accidentally.
Part of it is that we are close to the source, and used to manifesting our ideas directly into being. I agree completely. I will take it even one step further and note that if you work with a lot of spirit familiars, they can sometimes overhear even casual requests and have them come to pass. Even when the requests arent made by the magician, just by someone close to him. Say, his wife.
A few years back when we first moved back to NJ from Philadelphia, my wife used to tool around in my mother-in-laws volvo. It was old and somewhat unsafe. My wife turns to me and says:
"Cant you get your 'friends' to make my mom buy a new car".
30 minutes later, after wife drops me off, the car breaks down for good. Mother-in-law had a brand spanking new car by the end of the week.
A couple year or so after that, after a dinner at my mother-in-laws house, which was going to pots a bit and in a neighborhood that was getting a little sketchy, my wife says:
"Couldnt you get your 'friends' to get my mom to finally buy a house? This one is falling down and I hate to see her waste money renting it..."
Three months later Mother-in-law gets a letter in the mail saying the house will be taken by immenant domain and that even though she is a renter, they are going to give her money (a shitload of money in fact) to move. She buys a house, where she lives to this day.
There are more examples of this, but as a magician you need to watch what you do and say. Even placing things casually on an altar can have consecquences. There was a time when i still lived with Brownlee in philly that I left a piece of paper from an art project with the word "psychedelic" on it. The next day a friend was kind enough to drop off a bunch of other, smaller, bits of paper that had been soaked in... lets say a magickal oil called Love Saves the Day...
Anyway, be careful of what you wish for, or even joke about wishing for.