Very hard to convince me a BULLET aiming for the president's head deflected off a leave (which by all accounts is not supposed to be strong enough to deflect a bullet).
Bullet hitting a curb and knocking a piece of rock/glass and hitting an onlooker? Plausible, i guess.
A bullet deflecting off a tree leave? total 
Just to be accurate, Posner's theory is that the alleged lone gunman's first bullet hit a
limb of the intervening oak tree -- not a leaf, but a limb. Then, so the theory goes, the bullet's jacket separated from the lead core, and the bullet then sent two large fragments streaking in different directions, one striking the pavement behind the limo and the other magically weaving through the other tree limbs and jetting straight to the Tague curb site over 400 feet away.
Yes, it is fairy-tale material to assume that a supposed "sharpshooter" aimed at JFK's head or upper back but was so wildly off the mark that the bullet hit one of the tree limbs, and even hit far enough up the limb so the limb didn't snap or bend. No sensible, rational, objective person is going to buy such an utterly absurd scenario.
Specter and the FBI recognized immediately that the Tague wounding wreaked havoc on their single-assassin theory, which is why they ignored it for as long as they dared. Indeed, as mentioned, when the WC finally asked the FBI to investigate the Tague incident, the Dallas FBI claimed they could not find the Tague curb scar and theorized that street cleaners had wiped it away!