I just noticed something:
When you toggle an event with an else branch (something like the example below) it only toggles the first event. So it leaves you hanging with an untoggled else branch. Then when you try to run Construct complains about the hanging else branch.
+ variable lower than 3
So maybe when you toggle an event the else branch would also be toggled. I guess this logic would apply to any type branch that Construct doesn't want left by itself.
I hope that made sense.