Words for similar but distinct concepts, that are not themselves similar. The ELF standard for object and executable files contains two concepts which are similar enough to confuse, but different enough that it's normally important not to confuse them, and they're called ‘section’ and ‘segment’. I often wish they'd been called by more obviously different names: ‘section’ and ‘kangaroo’, or something. And I was just reminded this morning of another similar case: ‘project manager’ and ‘product manager’ as distinct corporate roles.

If two concepts are similar but distinct, the words for them should not reflect this by also being similar but distinct! They should be as different as possible.

Moral versus probabilistic ‘expect’. This might fall into the same general category as yesterday's moral vs tactical ‘should’, though I'm not sure whether ‘probabilistic’ and ‘tactical’ are similar enough for it to count. But even if so, it's a particularly noticeable sub-case of it and worth mentioning in its own right.

Imagine a parent saying to a child, before going to visit someone for the day, ‘Now I expect you to be on your best behaviour’; and then, when the child has left the room to get ready, they turn to their co-parent and say ruefully ‘I expect him to throw a huge screaming tantrum, so we'd better be ready to leave in a hurry’. Two clearly distinct words for ‘expect’, please!