Custom fields that are not simply text (eg. Version and Project Picker types) do not display correctly in the change history when edited. The database ID shows, instead of a human-readable value. Eg. the change history when changing a Version custom field value:

Scheduled Fix Version 10000 10002

and in an email notification:

Test User updated TP-20:
------------------------

Scheduled Fix Version: 10002 (was: 10000)
ProjectPicker: 10000