Some actions return a List of items as output, instead of a single item - this happens even in cases where the list only contains one item.
Attempting to use the output of such actions as a single item will produce the following error message:
Wrong value passed into ... Cannot convert a List of <Variable_Type> into a <Variable_Type>
In order to use the output of such actions, you will have to treat them as lists.
For example, a "Retrieve Emails" action will produce a List of Email Messages as a result, even if it only retrieves one message; in order to refer to that message, we would have to use %RetrievedEmails%, rather than %RetrievedEmails%.
Actions which return lists as output include: Get Files in Folder, Copy/Move/Delete Files, Get Subfolders in Folder, Email retrieval actions etc.