The Special:MergeAccount page is used to connect unattached local accounts to a global account using the password of the local account and potentially auto auto-merging if the user accounts share an email address.
Temporary accounts do not have passwords or email addresses, and therefore any attempt by a temporary account to use this special page will not be successful. As such we should consider disabling access to this special page for temporary accounts.
An example of this special page being accessible by a temporary account:
