When hovering over a dropdown list, it becomes active and an accidental move of the mouse wheel can change the value of the dropdown box without the user being aware of it occurring. This is a problem with mouse wheels that can spin freely as it is much easier to mistakenly nudge the wheel when moving the mouse. I have done this multiple times and blamed Linux for messing up the settings before I realised the real problem.

If a user wants to change the dropdown list they should have to click on the dropdown and then use the scroll wheel. When dropdown appear in a column (like when editing entries in the system tray [picture attached]) it can be much more of a problem as there is a larger area in which to accidentally change a value.