Thanks, Russ, for giving me the first hit (for free!) now I'm addicted to hunting down and acquiring these little peculiarities. It baffles me that I get such a charge out of hunkering down over a loupe looking at these tiny little details, but I do!
First, here's a 90L1L double transfer. Described by Richard Doporto as a
"massive" double transfer, just about everything on the left side is doubled.

A carriage return and a line feed brings us to 91L1, a
triple transfer, one inverted. This example is unfortunately a so-called dry impression, so I'm still on the hunt for better examples, but the price was right and I'm just excited to have one.

For this stamp, all the action is happening in the lower left corner. The 2 below is pointing to the second, inverted transfer. That "O" is from the O in Postage. The 3 is pointing to the third transfer.

I took a scan of an ordinary stamp I have, cut out Ben's profile, flipped it over and superimposed it on the detail. I don't know why, but to me this is so cool!

And then the third transfer. It's the right side up, but shifted four tenths of a millimeter to the northeast.

Edit: fix link to slingshotvenus