My understanding is that the whole point of handwritten signatures is that they serve as evidence of a person's identity. Given that, it's critical that peoples' signatures not change drastically from signing to signing.
From the looks of it, Treasury Secretary Jack Lew's official signature, soon to grace paper currency, is simply not valid. Just compare it to the signature he's been using up to now:
Credit goes to Rebecca Leber for noticing this alarming development. Update: It appears the Wall Street Journal was the first to notice the plot at work here. Kudos to their team, especially Jeffrey Sparshott.