The number one spelling error that draws the ire and/or ridicule of everyone who knows better is writing it’s when the context calls for its.
Incorrect: The dog is wagging it’s tail.
Correct: The dog is wagging its tail.
There’s an easy way to keep this error from creeping into your written work.
Don’t EVER write it’s.
At least, not in your first draft.
If what you mean is it is, write it is.
If what you mean isn’t “it is,” write its. NO APOSTROPHE, EVER!