About a sim week later, a "divorced" sim appeared with two twin babbies. I took control to check their family tree and they do not show a baby daddy. I think this may have occurred because I was playing around with Buzzler's Birth Control Mirror {...}
And you're probably right. It's almost definitely my fault. I checked my code and while unlikely it's possible that the code starts a pregnancy without a valid reference to the daddy (the pregnancy handle references the Sim class in the code, i.e. the "Avatar"; what really defines a Sim is the SimDescription class, which is what I'm using when looking for victims; there's a reference from SimDescription to Sim, which has to be checked whether it returns a valid object; the check is actually there, but its reaction is borky) .
Pretty embarassing mistake too, I have no idea how I missed that in the first place. Sorry about that. I'll fix it right away.