As documented in each, it is safe to use the each iterator to iterate over a hash and delete the element most recently returned. It is also safe to generate a list of keys, and then delete hash elements by iterating over those keys -- you're not deleting elements from the list returned by keys, you're deleting elements from the hash that were listed by keys; a subtle but important difference.

With an array, deleting elements requires rearranging the array, which (as the C++ folks like to say) invalidates the iterator, or as you mentioned, the array gets "completely messed up."