English may not be a particularly difficult language to learn, like Finnish or Arabic, but no other language has a frustrating amount of words that sound the exact same as other words but mean something different. Curse you, homonyms! Yes, we even created a word for our words that sound the same as other words, homonyms. They as well as some of our little grammar idiosyncrasies make English a little bit frustrating to those trying to learn it.
However, you do not have to be an English as a Second Language student to make mistakes with it. Homonyms are tricky. Especially when you learn a phrase by hearing it opposed to reading it. These phrases can often end up misheard especially if you live in the Southern or Midwestern bits of America where everyone is afflicted with a lovely little speech impediment known as "a twang."
A lot of people spend their lives misusing these commonly confused phrases, at least until some jerk comes up and corrects them. Let me be that jerk.
"In the mist of" or "In the mits of"
I believe the phrase you are looking for is "in the midst of" which means that you are in the middle of something. This phrase and the other two that are often used in place of it are known as a mondegreens, which is a term used when someone misinterprets a word with another that sounds close to it.
"In lame man's terms"
The proper term is "in layman's terms." This is probably the most misused phrase that I hear the most. In layman's terms refers to taking something complicated and explaining it a way a layman or common laborer could understand it. In lame man's terms would suggest that people with wounded or warped legs have some kind of special language.
"Once and a while"
The proper phrase is "once in a while." Once and a while, if you look and think about what those words mean, makes no sense at all. Once in a while means that you do something every so often. However once and a while means you are going to ... I do not even have a good or funny example as it really makes no sense.
"Reap what you sew"
The proper phrase is "reap what you sow." This is a classic case of homonyms inflicting horrible havoc. However, unless written, most people will not even know you are using it wrong because it sounds pretty much the same. Reap what you sow means that you harvest what you plant. Usually it is not used in an agricultural sense. It is more like, if you punch someone in the nose you deserve it when they hit you back. Reaping what you sew suggests that when you are done sewing up your pants, you are going to harvest the crap out of that denim.
"Peak your interest"
The proper phrase is to "pique your interest." To be fair, both are correct however most people confuse it because "peak" and "pique" are homonyms. To pique one's interest means to arouse your interest or get you interested. Peaking a person's interest means you have gotten their interest as high as it can go. Again, both are right, but when people say it, they usually mean pique.
"Never seizes to amaze me"
The proper phrase is "it never ceases to amaze me." This term means that whatever someone is doing or saying never stops (ceases) amazing you. Never seizes to amaze me would refer to the topic in general never taking possession of your amazement. Usually this phrase is misused purely because when people learn it, they misheard it as if you pronounciate well enough "seizes" and "ceases" are fairly distinguishable.
The original phrase is "deep-seated belief", however this is really quite heavily argued. Both mean about the same thing, however the original phrase is deep-seated which referred to horseback riding where you are firmly placed in the saddle and hard to move. The phrase deep-seated belief means that you are very firm and unmovable on a subject. Of course, deep-seeded can mean the same thing, you are planted deep in the ground, firm and unmovable-like. This is one of those phrases that have just kind of mutated, both sides are right but only one came first. Talk about being deep-seated, man.
"Nip it in the butt"
The proper phrase is to "nip it in the bud.” Nipping it in the bud is a gardening term which means if you cut off the bud of a flowering bush, it will not flower. This is usually used outside of gardening when people are talking about stopping a certain action or behavior. Biting someone in their fleshy tushie is not quite as effective as an action deterrent ... or is it?
"I could care less"
The proper phrase is "I could not care less." This one is pretty obvious. To say that you could care less means that you do indeed care, but it could be less. To say I could not care less means you are at your maximum amount of not caring and there is no feasible way you could care less than you do now. When learning this phrase, most people hear it wrong because of the contraction "couldn't", a lot of people forget to annunciate it when talking, so thus this weird mutant offshoot is born.
"Without further adieu"
The proper term is "without further ado." This is another phrase that you would not know someone is using wrong until they write it. Adieu is just a fancy way for posh talkers to say goodbye and ado means fuss. So thus, the misnomer up there really makes no sense unless you intend to leave in the middle of a sentence. My favorite way of remembering this phrase and its proper usage is to recall Shakespeare's play Much Ado About Nothing. It was hilarious.
"Outer body experience"
The phrase for what you are having is an "out-of-body experience." We have outer body experiences every day. They are nothing special, especially when we have some friends who just do not know how to keep their hands to themselves. However, when you are floating up top at the ceiling looking down at your unconscious body, that is something special!
"For all intensive purposes"
The phrase you are trying to say is "for all intents and purposes." Ho boy, I have met some people that absolutely get furious when this bad boy is misused. The phrase started when King Henry issued the Statue of Proclamations in the 16th century. In which it contained the phrase "to all intents and purposes", which was just a fancy-smamsy way of saying "I can modify this whenever the heck I want." It has since morphed into the current "for all intents and purposes" which essentially means "in effect." "For all intensive purposes" is the product of drunk people slurring their words together (probably).