I think this should be an assert in nodeAddSocket. An additional check in rna code seems appropriate. There you could also check for group inputs and group output nodes, I think. Does this throw a Python exception now?

For me it looks inconsistent that frame node can have input/output collection because it actually can't to add there anything.

Yes, the decision that frame nodes are just normal nodes is a bit weird, maybe I'm missing some historical context. That's how it is for now though...

In D7671#184819, @Jacques Lucke (JacquesLucke) wrote:

I think this should be an assert in nodeAddSocket. An additional check in rna code seems appropriate. There you could also check for group inputs and group output nodes, I think. Does this throw a Python exception now?

Atm., you can create inputs on Group Input nodes and outputs on Group Output nodes...
Creating outputs on Group Input nodes and inputs on Group Output nodes fails silently

What does it do when you create a new output socket on a Group Input node now? Does it return None?

In D7671#184869, @Jacques Lucke (JacquesLucke) wrote:

What does it do when you create a new output socket on a Group Input node now? Does it return None?

returns NodeSocketVirtual