When I’m reading, one thing that’ll jolt me out like nothing else is anachronisms. Please do your research and make sure that nothing is out of place/time.

Time itself is a prime example of anachronisms in stories set in the past or in fantasy settings inspired by the past. In a medieval setting (which is also still very popular in fantasy), even though there are clocks in church towers or secular public buildings, people are unlikely to carry a time piece around with them and say “We’ll meet again in 30 minutes” or even “Wait a minute”. (Unless otherwise specified in your fantasy world building, of course.)

Another example of an anachronism is the mention of electricity passing between the protagonist and the love interest in a book set in Ancient Rome. It jolted me so much out of the story that I haven’t read any of the other books by the author, even though I’m a huge fan of Ancient Rome stories.

You don’t want that to happen with your readers, do you?