I'm told that when you try to "hotswap" a set of classes into the VM and one or more of them can't be swapped in for some reason, the whole swapping effort is cancelled (that is, the redefinition of classes is done atomically).
This is great!

However, I'm also told that information is not delivered to the debugger so it can tell the end user which class(es) couldn't be redefined or why it wasn't permitted. It would be really nice if we could give the user some meaingful information about why their action wasn't permitted (it might even help them understand the feature better which might make them use it more)