EOS Rebel shutter half opening

I was thinking one of two things here:
1) Flash sync speed, if you are using a speed above 1/200 and your flash is firing, it will produce an effect similar to this.

2) shutter failing, usually this initially shows up as an arc across the frame then progressively gets worse until a blade or two completely fail and renders something like your example.

If you put the camera in cleaning mode the mirror will be out of the way and you may be able to see your shutter trouble.

It's not a broken shutter, it's a broken hinge on the submirror. Common failure in the 300D.

You can see for yourself - take the lens off and set the shutter speed to 5 seconds, then trip the shutter. The mirror will go up and the submirror is supposed to fold up behind it. Bet it doesn't.

There's a thread on POTN by a guy who fixed his 300D, but it involved some creative surgery.

-js