Non-citation of basic knowledge
I find Latour and Woolgar's spectrum of "facticity" a useful tool for thinking about these questions (a nice summary can be found at this link).  It breaks scientific statements into five rough categories by level of certainty in the assertion:


Speculations - don't have to be backed by anything
Descriptions - not established, so need to be directly backed by evidence
Tentatively established - need to be backed by citations
Well accepted - should be stated, but don't need evidence or citations
Tacit - should not even be stated


Where exactly a fact lies on this spectrum depends on the community and state in discussion.  In general, the broader the audience, the less well accepted facts should be assumed to be.  I think the notion "Should everybody reading this have been taught in a class?" is a good one, though undergraduate is not necessarily the stopping point.  For a machine learning audience, for example, you should assume everybody has had graduate level machine learning courses, while for a biology audience you should not assume they have even had undergraduate computer science.