Three cases of thumbnails not being displayed were reported in the following comment - all three reported cases are on commons wiki:
Steps to reproduce:
- follow the links above and have the number of search results on a page to be set to 500 (or, if all results get displayed properly, click on the Next 500).
- while a page with search results is loading, there are no indication to a user that the request was submitted
- when a result page loads, some thumbnails image won't be displayed
e.g. for the above example (1)
- the Console will show
GET scheme https host upload.wikimedia.org filename /wikipedia/commons/thumb/7/71/Handbook_of_ornament%3B_a_grammar_of_art%2C_industrial_and_architectural_designing_in_all_its_branches%2C_for_practical_as_well_as_theoretical_use_%28IA_handbookoforname1900meye%29.pdf/page1-120px-thumbnail.pdf.jpg Status 429 Too Many Requests Version HTTP/2 Transferred 2.88 KB (1.78 KB size) Referrer Policyorigin-when-cross-origin Request Priority Low