I thought that I would compile a list of mistakes that I've heard people make in davening. Here are few that come to mind:
1) The Praises of India: Hodu - means "praise!" when the second syllable is accented. Unfortunately, many put the accent on the first syllable. Then the word means "India"!
2) Praise vs. Splendor: When returning the Torah scroll to Aron HaKodesh I've often heard people say "Hodu (shuruk instead of Cholam) Al Eretz VeShamayim" (meaning "praise about the heaven and the earth") instead of "Hodo Al Eretz VeShamayim"(His splendor is upon the heaven and the earth).
3) Uber Chochoms: In the third "yehi ratzon" after the Torah reading on Monday and Thursday I've heard "lekayem banu Chochmei( as if there is a kamatz katan under the chet) Yisrael" instead of "Chachmei Yisrael". You got to be a real uber chochom to turn a patach into a kamatz katan.
4) The Grammar "Experts": This one is really weird: Shabbat morning in the kedusha of shacharit: "Kadavar Heamur(as if there is a segol under the hey) instead of "Kadavar Ha'amur."
If anybody out there knows of any other common errors, leave a comment. Maybe I'll learn about a mistake that I make. Thanks.