This is a bug that as far as I can tell is still present in 10.8.5 and has been around for awhile.
Here's what to do:
- Open Mail. File> New Message
- Immediately set your Signature to [None] (Yes, your Signature)
- Now, click the Templates button
- Scroll to the bottom and select [Custom]
- Choose one of your custom templates (It will work now)
Now that you have found this workaround, PLEASE
and report this bug.
Apple will probably not respond to these posts. Make an official report!
Be sure to select: Feedback Type: Bug Report
Feedback Area: Mail