Indeed, "The only man who makes no mistakes is the man who never does anything."
And since we are all here to learn from our and others' mistakes, please don't be embarrassed to share stories. It's for the greater good.
I'll start with a really embarrassing mistake I've done, myself!
The word: "Gengo!"
I spent over 2 years pronunciation the syllable "Gen" in the same way it's pronounced in the words: General, Generous, Gene, Genni ... etc.
My excuse was that all text-to-speech programs I know of did pronounce it in this way!!
I found out about the proper pronunciation 2 years later from Gengo's Youtube channel (https://www.youtube.com/user/gengo). Still, I was accustomed to the wrong pronunciation that I even used it during an interview with the UX designer at Gengo "Mr. Richard Hatton" and I was so embarrassed!!!
The lesson I learnt from this mistake is that; no matter how universal a rule might seem to be, there's always an exception! So, never take something for granted. And always be on guard! :)
It's your turn now...