To reproduce:

• Drop a playable / embeddable video file into a comment.
• Continue typing text, keep an eye on the preview.

Actual behavior:

• In Safari at least, preview is a flickery mess as the video constantly resizes / loads / unloads.
• Safari eventually warns you that the page is using "significant memory" and will doom civilization.

Expected behavior:

• Not that.

Possibly: embed a "cover image" instead, or make videos-in-previews work differently from videos-in-published-content.