While going through reviews on AnkiWeb this error comes up frequently after maybe 5-15 items:
Is this because of an error in my collection (a missing media file maybe?) or a bug in AnkiWeb?
Perhaps some JavaScript on your card template is causing it?
Yes, seems like it was a combination of a missing media file and some JavaScript code of mine. The JS code changed the innerHTML of an element to include a reference to an image that was not uploaded to AnkiWeb: <li style="list-style:url(_Contra.png);">