I browsed a category using an obscure setting ( https://commons.wikimedia.org/wiki/Category:Uploaded_with_en.wp_upload_wizard?gallerymode=packed ) that made it show thumbnails in sizes not previously generated. About half way down I started to get:
<html><head><title>Error generating thumbnail</title></head>
<h1>Error generating thumbnail</h1>
As an anti-spam measure, you are limited from performing this action too many times in a short space of time, and you have exceeded this limit.
Please try again in a few minutes.
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I would expect the "anti-dos" measure to only kick in when doing something significantly more extreme than what could possibly happen during normal browsing.