When a user mistakenly selects HRC20 for returning their ERC20 from Harmony to Ethereum, it is not prevented. I think the mapping check does not work here in this case, because the mappings are always one way: ERC20TokenAddr to HRC20TokenAddr. When the user selects HRC20, he inputs HRC20TokenAddr and the mapping check does not find any mapping, hence the check fails, resulting in mistake.
When a user mistakenly selects HRC20 for returning their ERC20 from Harmony to Ethereum, it is not prevented. I think the mapping check does not work here in this case, because the mappings are always one way: ERC20TokenAddr to HRC20TokenAddr. When the user selects HRC20, he inputs HRC20TokenAddr and the mapping check does not find any mapping, hence the check fails, resulting in mistake.