Names might do it I guess, though I can't really think of an example right now, but vocabulary can too. The example coming to mind (I'm fairly sure I've mentioned it before) is in the Dutch translation of one of David Eddings' books, where they use a certain expression involving the word "French". Needless to say, that rather jars with the fantasy setting. Still in Eddings, there's a scene somewhere in which characters discuss the word "defenestrating", which had the same effect to a lesser degree, because it made me think of the etymology of the word and of the Latin (and of course the Defenestration of Prague). And I seem to recall it happening a few times in other fantasy series too (definitely in Jordan, and Terry Goodkind has a place called "Nicobarese" somewhere, after the Nicobarese Islands in India).