TREE(3) = A number so big that if you tried to memorize it, all the information in your brain would literally make it collapse into a black hole (and yet, it's still a finite number)
FSIX threw me for a loop until I got SEBASTIEN (I didn't know FLIBBERTIGIBBET but that's a fun entry). At least it's not a mean clue like "Chess move that Grandmaster Ben Finegold says to never play" (seriously, never play f6)
Yeah, "extremely" is a bit of an understatement, isn't it :P
As for FSIX: that was my least favorite entry in the grid, due to the fact that there was nearly no way to clue it universally (except for "key in the middle of the top row," which is... bleh). The most agreed-upon function of FSIX was moving the cursor to the address bar, but even that isn't completely agreed upon from keyboard to keyboard. For example, on the Surface that I'm typing this on, it highlights tabs. Thus, if I'd thought of using a chess clue, I probably would have.