Create a new button on a toolbar, completing both the label and tool-tip fields. When the mouse pointer is over the button, the tool-tip pops up and shows the label text as well as the tool-tip text.
If both fields are filled in, the label text has a ':' appended.
In fact the bug appears in the default toolbars as well. If you hover over a button, you should see two lines of text in the tool-tip if both the label and tool-tip text have been filled in. If only one field is filled in, then that field is displayed (without the appended ':' in the case of the label field.
I have shut down DOpus, rebooted and can reproduce the problem every time.
The tip field is for additional information that clarifies details that wouldn't fit in the short label. If the button's label is hidden (as in your example, where it looks like the toolbar itself has labels switched off, so the button's Show Label checkbox is overridden), then the label is included in the tooltip (else you would not see the label anywhere at all), and the extra tip information (if any) is added on another line. The label lets you know what the button is called and gives a quick idea of what it does, while the extra information from the tip field can explain things in more detail.
If you look at the tip fields on buttons from the default toolbars, you'll see they have explanatory sentences in most cases.
