Three of these four number pairs obey the same rule. Which pair does NOT obey it? (5, 26), (7, 50), (9, 80), (11, 122)

The rule is 'multiply the number by itself and add 1': $5\times 5+1=26,7\times 7+1=50$ and $11\times 11+1=122$ all fit. For 9 the rule gives $9\times 9+1=82$, not 80, so (9, 80) is the odd pair. The trap is (11, 122), which many pick just because its numbers are the largest; the trap inside 80 is that it looks close enough to $81=9\times 9$ to slip past an unchecked eye.