If you'd really meant DO, the verb, then you'd have written "...transactions fail; in 65 % of the cases, because of what user's fat fingers do...", or something like that. With the word "do" _after_ the "fat fingers", not between the consequence (transactions fail) and the reason (fat fingers).
But you didn't; you wrote: "transactions fail 65% 'do' to fat fingers", with "do" *between* the consequence and the reason -- the EXACT structure you'd also use in the correct sentence (fragment), "transactions fail, 65 % due to fat fingers".
So FERFUCKSSAKE, don't try to *weasel* your way out like that! Sheesh, man -- it's one thing to be sloppy with language (and that's bad enough), but not to stand up for it like a man...? I certainly thought better than *that* of you!