Platform: Linux.
Product: Progress Telerik Fiddler 0.5.0, downloaded today.

Bug 1: switching between Composer and Inspectors clear Composer fields
Start the program
Click Composer on the right side
Under Headers, fill something in
Click on Inspectors on the right side
Click Composer on the right side
The text under Headers is now resetted

Bug 2: POST body double new line delimiter and body data are formatted as headers
Start the program
Click Composer on the right side
Select POST

Under Headers, fill in a POST header:

Host: example.com
User-Agent: test
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US;q=0.7,en;q=0.3
Content-Type: application/x-www-form-urlencoded
Connection: keep-alive

Under Body, fill in some POST data:

data=1

Click Execute
Click on Inspectors
Click on the result request

Check Headers or Raw, it says:

POST http://example.com HTTP/1.1
Host: example.com
User-Agent: test
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US;q=0.7,en;q=0.3
Content-Type: application/x-www-form-urlencoded
Connection: keep-alive
:
data=1:

Notice the ending, it handles the POST data as parts of the header