Steps to reproduce:
- Create a feed mapping with a slash in the source name, for example "One/two"
- Create a feed tamper for this field mapping (I used explode)
Tamper appears in the table under the "One/two" mapping on the plugins page.
Tamper does not appear. I can see the mapping in the feeds_tamper table, but the tamper is never passed to theme_feeds_tamper_ui_list_form (that's as far as I got debugging).
Don't use slashes for source names in your feed mappings. It doesn't seem to matter if there is a slash in the feeds target field name.