Not sure if this is a bug, but the deathclaw that spawns in Old Gullet Sinkhole is never aggressive. it just sits there and stares at me. the first time I discovered the location, he was aggressive. But he is nonaggressive every other time I have went there since then.

I don't know about this specific deathclaw but I have run into "friendly" feral ghouls, raiders and triggermen on occasion. I'm pretty sure its a bug.

i know there is one instance of a friendly deathclaw during a mission where you return an egg to its nest, not sure of the location, but maybe that deathclaw is always friendly as its part of that quest?

From the Old Gullet wiki entry noting the bug...