Anecdotally I've had to untangle lots of half applied state. Sometimes a provider has a bug, sometimes somebody on the other side of your company changed your terraform plan without migrating the running state, sometimes there are network issues that prevent your plan from applying correctly, sometimes the cloud provider said it deployed the resources but it actually didn't, etc, etc.

It's extremely painful and delicate to do and it may not happen frequently, but in my experience it happens frequently enough when I'm dealing with a lot of terraform.