Ok I'll add that. Done.

I think cause when I was writing it, my mindset was on expected should always = output of the method.
which is why I was surprised to see Expected = the original input cause my mind was like
"But the whole point of the method is to remove the smallest number!" that's why

Current test message:
You've mutated input array

Wonder if this will be clearer?
You've mutated input array (expectation assertion is on value of input array, not output of method)

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]

Maybe the test expectation message can be more accurate? I thought the expectation was the output of the method, which was what confused me.

ahh right thanks - i took that to mean don't mutate the order

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.

Sorry should have been clearer - this is with the mutations to original array test

> check for mutations to original array
Log
Direct input is [147, 376, 340, 233, 281, 322, 132, 128]

You've mutated input array -  
Expected: [147, 376, 340, 233, 281, 322, 132, 128], 
instead got: [147, 376, 340, 233, 281, 322, 132]

What I don't understand is: If the direct input is [147, 376, 340, 233, 281, 322, 132, 128], I would expect the smallest number '128' to be removed - but it seems the expectation does not expect this

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.