Woop, 11 down!
[much later]
Welp, I got 13 of them.
If you don't recognize the sequence: "6, 28, 496, 8128," don't waste your time trying to figure this one out! (There's at least an hour of my life I'll never get back...)
"14, 23, 28, 33, 42" - _NOT_ outside the box!
The author of the site thought he was being clever with this one, but I beat him!
The +2 / -2 is the key...
14 = 7x2
23 = 7x3 + 2
28 = 7x4
33 = 7x5 - 2
42 = 7x6
51 = 7x7 + 2