[Internal Error] Two progressive infos can't exist together

Haven't been able to reproduce this yet, but I remember locking preview on two different components in quick succession when this error occurred.

[ Edit: Damn, the forum software does not let me attach the .html bug report file (too big & wrong file type). Can't attach zip either, and changing extension to .jpg or .png also does not work. ]

[ Edit2: GMM, I've submitted the bug report via the web form. It is about 300000 characters long, so in order to submit it here I'd have to split it up into multiple posts of 48000 characters for each post. This seems a bit inconvenient, if you ask me. ,) ]

Alright, I'll refrain from posting the bug report here then.

But what exactly is a progressive info, I wonder...

... and - more importantly - why can't it be together with another of its kind?