I was trying to upload this image when asking an android-app question, but I got the following message when pressing "Add picture" after selecting "From my computer" and browsing the file (you need to download the image first to repro):
For security reasons, framing is not allowed; click OK to remove the frames.
Then I clicked OK and presented with a question:
You have started writing or editing a post.
Are you sure you want to leave this page?
If I press "Stay on this page", the dialog continues the "Uploading..." animation.
If I press "Leave this page", I'm redirected to your This is our fault error page.
I would expect you to say the file is too big or something more pleasant describing the problem, not redirect to a generic 503 page.