I believe I've already shared this here, but the worst one I've ever had was "I hurt my knee today" being corrected into "I hit my nigger today". It was terrible. I don't even use the N word, I have no idea why it thought I was going there!

My most recent one was "I will jump in the lake with you" corrected to "I will hump in the lake with you", texted to a girl who I knew liked me but I wasn't really that into. Her response was full of eagerness and I didn't notice the auto-correct mishap until much much later. :x