I dug around in the OED. Here's what I found: EDITED AFTER READING COMMENTS
1. To be cautious or on one's guard, to be wary; to take care, take heed, in reference to a danger.
b. with of (from, with, obs.): To be on one's guard against. <-- (EDIT) partially obsolete?
c. with infinitive. Obs. <-- obsolete
d. with clause: lest, that not, how.
e. with simple object; = 1b. <-- (EDIT) not marked obsolete, but PART of 1b is
2. To take care, have a care of:
a. with of. Obs. <--obsolete
b. with simple object. Obs. <-- obsolete
c. with infin. or clause. arch.
3. To take warning by. Obs. <-- and once more
So it seems that the only definitions that are not marked obsolete are 1a, 1d, and 2c. Now, 2c might not be marked as obsolete, but it is marked as archaic, so let's ignore that one. (EDIT: Also, it seems I was reading one of the Obs. wrong, so let's add 1b/1e back into the mix.) This leaves us with the following examples:
1d. Beware how you indulge. Beware that you don't get caught.
1b. Beware of getting lost. Beware of self-deception.
1e. Beware the dog!
So, after looking at this list one more time, it seems that there are a few options that are not yet obsolete, yet we don't hear being used very often. This might lead to them sounding wrong, even when they are not. The same thing has happened to the word "whom," with many people believing that you're trying to sound smart when you're only using it in its proper place in the sentence. (Sometimes even I have to pause to make sure I'm not hypercorrecting before asking someone whom they are looking for.)
Thank you, comments! I must indeed beware of parentheses!