I found an discrepancy in the behaviour of (O)MPT :
In the sample- and instrument-tab, if one wants to select one, one can increase the number by clicking on the small 'up'-button at the right of the number.
Now, being in the Pattern-tab, I suddenly have to press the small 'down' button to increase the number of the Main instrument.
This is because you use the arrows to navigate in the combobox. In the other cases, there isn't a combobox, and up for higher and down for lower is more "natural".
In my opinion, the "right" thing to do would be to remove the arrows by the combobox, as I don't think it's standard procedure and will only serve to confuse in the long run.
The combobox indeed makes the difference, you're right. And the solution to the confusion is excellent. A combobox normally has one big down-arrow button aside, to open up the list.