Book of Abraham
This one was a real biggie for me. The Book of Abraham is a book of scripture that is part of the Pearl of Great Price. It is official, canonized LDS scripture. It came to be when Joseph Smith purchased 4 mummies and some Egyptian scrolls/papyri from a traveling salesman. Joseph Smith said that these were writings from Abraham, written by his own hand, and he translated them word for word into what is now the Book of Abraham in the Pearl of Great Price. He also provided explanations for the pictures on the papyri. However, every single scholar and historian and Egyptologist that has seen these have unanimously agreed that they have nothing whatsoever to do with Abraham or anything else in Joseph Smith’s “translation”. They were simply common funeral documents. They also date back to an era that is long after when Abraham lived. Joseph Smith got every single thing about them completely wrong. This is an example where we have an actual document that Joseph Smith “translated” which has been proven to be false. He very clearly made it up, which calls into question everything else that he supposedly translated, as well as his basic trustworthiness.
Want more detail? See https://read.cesletter.org/boa/, https://www.ldsdiscussions.com/book-of-abraham and http://www.mormonthink.com/book-of-abraham-issues.htm