Opera displaying images as text
Here is an interesting incompatibility I found with the Opera browser. If you create a custom HttpHandler that takes a URL like http://mysite.com/myhandler.axd and internally does a Server.Transfer to a JPG image, Opera will not recognize it as an image and will display it as text (and I don't mean ASCII-art). It only happens in Opera. Other browsers work just fine.
I find it rather amusing, since in theory, Server.Transfer happens on the server before anything is sent to the client. So after Server.Transfer the browser should receive the correct HTTP header along with the image. It should be no different than requesting this image directly through IIS. Apparently there is a difference. It looks like the Server.Transfer doesn't change the content type of the Request, so you have to set it manually before calling the Server.Transfer method:
HttpContext.Current.Response.ContentType = "image/jpeg";
Now Opera will not complain!