Slopes have caused us several issue over time. Things are better than they've been before at the moment, but with the extensive re-writing of movement and collision code that's happening at the moment, the hope is things like this will be eliminated entirely. We'll see how things are once the new changes are properly implemented.
It something that's happened sometimes during a mission, but often it doesn't. I have seen it a dozen times in over a hundred mission attempts maybe? It's doesn't happen often but it's not that rare. Since it's one ant getting stuck it's not that big of a deal.