I've experienced a weird bug when working on #16401. When debugging the hash(::AbstractArray, ::UInt) method, I repeatedly pasted it in the REPL and called hash(::AbstractArray). It turns out that under some circumstances, the redefinition has no effect: the old method is still used.

I've been able to reproduce the problem with relatively simple steps:

1. start REPL in the Julia source directory and call import Base: hash, hash_range, TypeRangeStep, RangeStepRegular, hashaa_seed, hashr_seed, hashrle_seed
2. copy these lines to the REPL after adding a statement like println(1) at the top of the function
3. call hash([1]) and check that it prints 1
4. run using Test; include("test/hashing.jl")
5. copy the lines from step 2 after increasing the value in the println call
6. call hash([1]) and observe that it still prints 1

If you skip step 4, the call in the last step prints 2 as expected.