One of my more recent ones was that I drove into town with my wife, bringing my bike and child seat for it that fits on my rack. I also brought a pannier bag to carry my work clothes and my daughters day care supplies. I've never used the pannier before with the bike seat....and realized after my wife left in the car that the seat takes up the space where the pannier hooks on and it would not fit.

To fix this, I used some zipties to zip up the pannier to the back of the bike seat, hanging the bag hooks over the top of the seat.