"Cheatin' uh?" - what on earth is this? Creating custom HTML templates ...

Hi

I'm creating a custom HTML email template, and uploading revisions via FTP, and after the latest upload, I get this weird message - "cheatin' uh?", and now I can't edit the email in the plugin interface.

It seems to be built into the code, but it's not helpful\intuitive\informative.

How can I get rid of this and get on with my work?

Thanks
Richard