Hmm not sure probably baka as well but the one that impacted me more was probably ski desu (I like/ love you). 

I remember speaking to my Japanese teacher who wanted us to learn polite Japanese, so I had to repeat the following words over and over again  'O-genki desu ka.' It was funny, because later we had some transfer students over from Japan and my Japanese teacher happen to introduce them to me first.  The class was held late in the evening.  I looked at my Japanese book I had purchased to help me with my learning.  In the wake of trying to impress them and after a short pause I responded Konbanwaaaaaaaaaa.  To which my teacher looked puzzled and the transfer students looked on with a confused expression on their faces. Fond memories :)

The word for sea; Umi

Mine was "sayonara" with terrible pronunciation and I got it from the original Yours, Mine, and Ours movie starring Lucille Ball and Henry Fonda.  When I later figured out the pronunciation I laughing at exactly how bad it is BUT then I remember that most clueless Americans say it that way.  LOL.  The good news is it actually was used correctly as she never expected to see the guy again (bad blind date :D ).

Ohayou , the theme song of my all time fave anime Hunter x Hunter !!! I used to sing this even though the lyrics were incorrect.. somehow now, I am able to sing it correctly, only the chorus though... hahaha

arigato = Thank You