I was recently writing a scene in one of my stories that took place in a barn. My first inclination was to have one of my characters plop herself down on a convenient hay bale, since those have been commonplace in livestock barns since time immemorial, right? Well, actually ... no. I started to think. How is hay baled anyways? Is it something that people have ever done by hand? Hay bales, as we know them, are tightly compressed and bound together by something called baling wire. Would it be possible to produce something like that before sophisticated machinery, whether steam powered or hand cranked, started to make its appearance somewhere in the mid 1800s? A little research revealed that the hay press (or hay baler) wasn't invented until the 1850's, and the modern 'pick up' or 'square' baler wasn't invented until the 1940's. So those square hay bales that we all take for granted as a prop for square dances really didn't make their appearance until relatively modern times. In light of this, it is amazing how many fantasy novels that are set in ostensibly medieval or Renaissance era worlds make passing mentions to hay bales.
This is not to say that a fantasy world needs to parallel the history of western Europe in the timing of its inventions. Part of the fun of writing fantasy is the ability to mix and match from different times and places in history as well as being able to come up with things that have never existed at all. The expansion of the genre to include intentionally anachronistic amalgamations of different periods in history, with or without magic as an added catalyst, has allowed a variety of interesting settings, including all the variations of 'punk' that have been popping up. If you write a story in one of these settings, you are assisted by the presence of certain 'norms' and conventions that frame a reader's expectations.
But if you are trying to create a unique world that doesn't fit any of the pre-existing subgenres within fantasy, some thought may be in order as to what you're including and why. It is not inconceivable that someone could have developed an early prototype of a hay press in a pre-industrial society. After all, Queen Elizabeth had a flush toilet (as did some people in Ancient China and Minoan Crete). The Romans, of course, had bathhouses and aqueducts and fairly sophisticated sanitation, and the Bronze Age Indus Valley cities of Harappa and Mohenjo-Daro actually had very sophisticated running water systems. And the myriad of social and religious institutions that existed in the ancient world are simply staggering. However, a writer needs to be aware that many readers have a default expectation that a 'traditional' high fantasy setting will be essentially medieval.
If you are going to portray something in your world that some readers might question, it is probably a good idea to at least have it be something that the society could ostensibly support at its current level of organization. It is reasonable for a society to start using buttons as fastenings for clothes much earlier than our culture did, for instance, but perhaps it is less reasonable for people to have zippers.
So when deciding what sorts of day to day technologies and conveniences people in your world may have access to, don't be afraid to be innovative, but keep an eye to how you will make the presence of these things plausible to your readers in the context of the world and society you have built.
Image source: http://commons.wikimedia.org/wiki/File:Rectangular_hay_bales,_Fry%C5%A1t%C3%A1k_%281%29.jpg