Here are a few of mine:
In the 19th C., there was a popular song, "Let us smile be' your umbrella on a rainy afternoon." "Be'" is a contraction of "beneath." You folks probably already know how that "folk process" played out. The phrase now is, "Let a smile be your umbrella ... ."

I heard PP&M singing, "Weave Me The Sunshine," and played the videotape over and over, never being able to hear anything but
"We, we, be the sunshine after the pouring rain. We be the hope of a new tomorrow ... ." Didn't ever get it right until I saw the lyrics printed.

I heard "Michelle" the same way as was mentioned above, not realizing it was French (which I had studied). I thought they were singing, "Sunday mokey [something] play piano song, play piano song."

In Janis Ian's "At Seventeen, I sang the line, "...and death ensures equality ..." for years, till I saw it in the sheet music as "and debentures of quality ... ."

David/Bacharach "They say the sky, the sky's in love with you ... ."

I have definitely heard "Mt. Thyme" sung as, "build my love a tower," and thought it actually was "tower."

Joan Baez, Birmingham Sunday, sings "Young Carole Robertson entered the door... ." (But she put the accent on the second syllable of "Robertson." Without listening carefully to the song, I heard her sing, "Young Carol of Urtson ... ," and thought it was an old British folk song (though I did not know where Urtson was).