The distinction between French open-mid and close-mid vowels is often neutralized or unstable in certain positions. The distribution of the sounds also varies in some cases between dialects, so it's a pretty complicated situation.
You're right that in general, close-mid vowels, including ø, are associated with open syllables and open-mid vowels, such as œ, are associated with closed syllables.
However, there are exceptions, which can in rare cases lead to minimal pairs. As suizokukan said, "jeûne" and "jeune" are often given as an example of a minimal pair. The "French Corrective Phonetics Guide" from bonpatron.com also lists the verb veulent /vœl/ and the (rare) adjective veule /vøl/.