I always try to remember to use "cannot" when applicable as in "I cannot take an umbrella" (because I do not have one).
I also thought that the problem with "I can not take an umbrella" was that it was ambiguous: it could also mean that I could decide not to take an umbrella. Trying to write unambiguously most of the time, I would therefore never use it.
Now I see it claimed that it is in fact not ambiguous, and only carries the second meaning. Usually I would accept such advice at face value and start using "can not" when the second meaning is unambiguously meant, but this comes from a style guide I have other reasons not to respect very much.
Does "can not" have a single unambiguous meaning?
(I have read Why is “cannot” spelled as one word? but my question is more about current usage. The length of the answer to that question does confirm my prejudice a bit, though)