When you scroll down through the predefined objects. It moves the level I'm working on up and down depending on which way I'm going. It happens when I use the middle scroll key built into the mouse. It's not so bad in its self when its just one level but when I have the levels set up as one giant map it's very annoying.
/edit not that I use a lot of predefiend objects but I find it faster that matching tiles together such as cliffs etc..