Ok I'll add that. Done.

What's your suggestion? As you can see:

check for mutations to original array                           // This is what's been tested
Log                                                             // This log is part of your code 
Direct input is [147, 376, 340, 233, 281, 322, 132, 128]

You've mutated input array -                                    // This is the error message
Expected: [147, 376, 340, 233, 281, 322, 132, 128], 
instead got: [147, 376, 340, 233, 281, 322, 132]

It seems you didn't read/understood this part of the instructions:

Do not mutate the original array/list.

That test is only for testing input mutation, it's not like the others.

The ruby code is fine. I ran this to check the functionality as described.:

def remove_smallest(numbers)
  p numbers
  [42, 42, 42, 42]
end

And the results show that the input is not exactly the same as the expected output :

works for the examples
Log
[1, 2, 3, 4, 5]
Wrong result for [1, 2, 3, 4, 5] -  Expected: [2, 3, 4, 5], instead got: [42, 42, 42, 42]

works for random arrays
Log
[145, 395, 50, 206, 274, 5, 240, 140, 23]
Wrong result for [145, 395, 50, 206, 274, 5, 240, 140, 23] -  Expected: [145, 395, 50, 206, 274, 240, 140, 23], instead got: [42, 42, 42, 42]

Also, take note that there are 2,661 solutions in ruby for this kata, so any such bug would have been discovered long ago.