[megaman22]

The one that has been the bane of my existence is Lync/Skype for Business. So often, doing development or even just in mundane business scenarios, its worthwhile to be signed into multiple unfederated Lync pools at the same time, or just different accounts on the same pool.

I've worked with the underlying RTC libraries directly, or through the managed .NET wrapper, and there's no great technical reason for this; it's just that the Lync client has a global single-instance mutex.