Odd behaviour for TOPICPERMISSIONS

When editing a %TOPICPERMISSIONS% table in a topic, whether I remove myself or not I make I usually get the message:

"Are you sure you want to prevent yourself from being able to edit this topic?".

Then whether I get the above message or not after saving the next page shows the message:

Attention: Error saving topic. During save of SuperTopicName an error was found by the version control system. Please notify your Foswiki administrator. permissions action saved. Go back in your browser and save your changes locally.

The permissions change is successfully executed - I only get these two messages which I'm sure will confuse my users. Under some circumstances I only get the 2nd error, I can't quite put my finger on it.

Thoughts? The two messages may or may not be related... There nothing showing up in the Apache error_log during the transaction.

Here is a screenshot: