You mutated the input array

try to use print at some points in the routine to see if the program is doing what you are expecting it do

Even though 3 comes after the frist 5, 7 comes before the 2nd 5. Hence, 3,7 as a pair comes earlier.

Two of the excruciatingly long tests seem to be broken. Debug code:

puts "Initial ints length is: #{ints.length}"
if ints.length < 10
puts ints
end

shows that the "Two Million Numbers With Pair At End: Should terminate with a valid pair output" and "Two Million Numbers With Pair At Start: Should terminate with a valid pair output" tests are only passing in 2 and 1 element arrays respectively, but are expecting a match (that isn't present in the arrays).