Things go wrong all the time, even for the best of Witches. The first time I really messed up a ritual I was so embarrassed I wanted to go hide under the alter cloth. The God and Goddess have a sense of humor and have probably seen more mistakes than we can even imagine. If something goes wrong, set it right.
When casting a spell, observing a ritual or just preying, take care with it. Don't rush and keep calm. Also, always remember to practice before you do it. That will cut down on a lot of mistakes that are due from being uncertain about what to do next or whether you pick up the Athame before or after you light the candle. Practice, outside of a circle and make sure you don't actually try to do the spell. I prefer to leave out a few words or even just mime my actions. Decide how you wish to practice and work out what kinds of practice are most beneficial.
Kitchen witchery can often go wrong when the Witch does not focus correctly. The answer to that problem can be fairly simple. Meditation can do wonders. If you have problems keeping your purpose in mind or a stable emotional state, practice by meditating. Find somewhere quite and focus on an object such as a picture or emotion. Keep other thoughts or emotions from intruding and increase the amount of time that you meditate slowly. Another aid can be music. Music is a wonderful medium for conveying emotion and intent. Find a song that has words or music that reflect your purpose and play it as you work.
If something terrible happens, don't panic. Apologize to the God and Goddess or throw out the last batch of cookies that absorbed your anger at the neighbor's dog. If whatever you're doing can be salvaged, then do so. Don't let a minor or even major disaster get you down. If worse comes to worse, do a purification ritual and start from scratch. Sometimes starting from scratch can make the end result twice as effective.
Some of the most embarrassing 'oopses' I've made have to do with a ritual that was done at the spur of the moment. I was moving residences and I wanted to ask the God and Goddess for a painless move and say good-bye to my old home. Just as I was setting up, a Witch friend came over and we decided to do it together. Nothing seemed to work right. We both couldn't figure out what we wanted to say, we knocked down the alter candles and my friend's dress almost caught alight. Needless to say we laugh about it now but a the time we were very embarrassed.
Other mistakes I've made have been cooking adventures or while I was making batches of bath salts or hand creams for friends. Those were more easy to fix but it did make me aghast that I could make such silly mistakes. No matter how experienced we are, we make mistakes. Learn from them and keep going.
For more information see the 'Intent' page.