I know the following button code contains some nonsense but I'm sure you'll want to know about serious effects like a frozen viewerpane.
What I wanted to do was to create a button that toggles between a single display style with viewpane and a dual display style using @ifset. When toggling back to the dual display the last paths should be remembered. Since Set Dual=remember didn't work (shouldn't it?) I tried Set Dual=On,remember which caused serious problems. Here's the button code with which you should be able to reproduce the problem:
@ifset:viewpane=toggle
Set DUAL=On,Remember
Prefs STYLE=Dual
@ifset:else
Prefs STYLE=SingleThumbs
Show THUMBNAILSIZE 247
The dual path opened new tabs (no tabs should be present):
Closing the active tab resulted in this:
As you can see the display border is black and above the column header. Close the empty tab at the left display and you're back to single display.
The reason why this happens is because the command Set Dual=On was already set in the style and doubled by the button code. I had to clone my Dual style with disabled dual display option to make it work.
To further examine this problem I added a line Set VIEWPANE=On as 6th line to the above code. As you can see in screenshot 4 the result is a double viewpane. Closing one of them results in a frozen pane that is unresposible and always on top of the display:
My suggestion to avoid this sort of problems is to ignore style settings when they're doubled by a button code. Not every user is aware of all the settings in his styles. This way calling a dual style with a second line like Set DUAL=On,remember could work. Apart from this the command Set DUAL=remember should work on its own when calling a dual style.
EDIT: I uploaded the wrong screenshot with the frozen doubled viewpane. Screenshot 3 shows what happens after closing one of them. Here's the one with double viewpane: