sur or sure
In a couple of exercises it looks for 'etre sure' as the correct answer and rejects 'etre sur' as incorrect, yet 'sur' is used in several other examples. Is the example answer wrong or is this phrase a special usage & spelling?
No, you're missing the point. I am aware of the various adjective endings, but in the question there was no indication of whether the speaker was masculine or feminine, so I didn't understand why 'etre sur' was rejected in favour of 'etre sure'. In my case I was a man, so why insist on the final 'e'?
If you are sure that the other form should be accepted, report it (Report → My answer should be accepted).
If you are not sure, ask in the discussion for that sentence and not in the root Duolingo forum. You will most likely get answers from other members including native speakers.
There is a reason for it. French adjectives change their ending depending on the gender of the noun or pronoun. The basic rule is that the adjective gets an "e" for feminine and an "s" for plural.
sûr = masculine singular, e.g.: Mon père est sûr.
sûre = feminine singular, e.g.: Cette femme est sûre.
sûrs = masculine or mixed plural, e.g.: Nous sommes sûrs, Ils sont sûrs.
sûres = feminine plural, e.g.: Les filles sont sûres.
You may want to find and use some grammar book or site. Duolingo is great at practice and learning words, but it is hardly enough to figure out and learn all the necessary grammar.
Note that there is a special French section of Discussions that is more appropriate for such questions.