I think it would be pretty easy to register an accidental vote when I'm reading G2G on my smartphone, since the icons are so small compared with the size of my finger. Also, occasionally my computer operating system decides that I didn't really mean it when I told it never to follow the link where my mouse is hovering -- that change in mouse behavior could lead to accidental votes.
It's my practice never to issue "Down" votes, but I issued one accidentally once. If you have "voted" on a question or answer, you can see a symbol indicating the vote you recorded. When I saw that I had recorded a down vote, I skimmed the discussions I'd looked at recently, but I never did find the discussion where I left a "down" vote.