a somewhat special(tm) url field entry of the form url = {http://some.where/foo\_--\_bar} becomes http://some.where/foo_–_bar (the dash here being an en dash now).
i came across this while using jekyll-scholar but found the root of the problem to be here, i think. at first, i thought just deactivating the latex filter for the url field would solve the problem but then again \_ should be processed.