Bear with me while I try and explain / walk through this scenario.  I've tested this 100+ times, with every imaginable condition, and am able to reproduce this problem consistently.

Issue: Using Outlook for Mac 2011 on OS X Mountain Lion (10.8), when composing a new message, when a signature (plain text or html) is inserted, either automatically (default signature), or manually (via dropdown menu / button), if I continue to type where I left off and press the the space bar, the cursor / window will lose partial focus, and further key presses result in the system bell chiming.  At that point, I can click my mouse pointer back in the message body and regain focus.

My System:
Mac OS X Mountain Lion (10.8)
Outlook for Mac 2011 Version 14.2.3 (120616)

Steps to reproduce:

1.  Open Outlook for Mac 2011
2.  (If not done already) create a signature
3.  Create a new email message
4.  In the message body, type some text
5.  Insert signature
6.  Without moving the cursor, type some text that includes a space
7. Once the space bar is pressed, some elements of the window (i.e. font styles, paste icon) will lose focus / be greyed out.
8. Focus can be regained by clicking mouse into the message body.

This can be reproduced within the same message by simply inserting another signature.  Again following the same steps (6-7) the focus will again be lost.

Let me know if I can provide additional information on this.

Thanks,

Jaren