I use Glade extensively for a wide assortment of designs.  For the most part it is an excellent tool.  However, there is one behavior that drives me nuts.

Imagine scrolling through the Widget Properties pane using the mouse scrollwheel.
Wherever the mouse happens to be positioned in the pane at the start of scrolling is somewhat arbitrary, and the cursor doesn't move much during scrolling.  However, when the pane has scrolled to one or another end of its vertical travel, and the mouse cursor then happens to be positioned over a control like a ComboBox or SpinnerButton, the mouse scrollwheel scrolls through the values of that control instead of scrolling the pane!

A typical example of this is Opacity, which is specified for many widgets, and uses a SpinnerButton; I'm continually discovering this property to be set to some non-default value or another.  The designer has to be quite vigilant and doublecheck that none of his/her scrolling actions have caused properties to have become inadvertently changed.

That actually bit me in a bad way recently, when it was the Resize Mode combobox that was the victim of this behavior.  It became errantly set to Immediate mode, which triggered a GObject bug when this mode is used with certain containers, causing my whole application to crash.  It took hours of experimentation and isolation to discover the root cause, all because of this damnable scrolling behavior.

Since there is no way to distinguish between the various intents for the mouse scrollwheel, I would suggest disabling scrolling through values of these types of controls entirely via the mouse scrollwheel in the Widget Properties pane, or any pane that has such controls, for that matter.