Putting the bag right on top of the bars like in the picture could be risky if the beek is very clumsy or careless I suppose. But you’d have to bump the hive hard to get any to slosh out - it can’t just leak out on its own because a vacuum is created when the air pockets move out, which occurs spontaneously when the cuts are made in a well-filled bag. Sometimes there’s a stray air bubble or two that easily escape with gently pushing it toward the slits. An easy safeguard is to just put it on top of the inner cover, which is what I do.