Today's royalty-free Stock image sites are relatively open, meaning that theoretically, anybody can submit any kind of content, including copyright-protected content created by others.
It's even happened to myself: I accidentally uploaded a photograph to a german Stock photo site once, only to discover later that it wasn't my own photo at all, but one I had downloaded from another stock image site!
Using content that was uploaded this way to an open stock image site could, theoretically, have problematic consequences for both the designer and their client. Although the content was used in good faith, the third party could make claims, as their work was used without permission; it will most often be impossible to shift the blame to the stock content site one got the image from.
Is this an issue at all?
How to protect against this kind of thing? Is there any protection?
Or is it simply an unavoidable risk when using content from free or cheap stock imagery sites? Is it necessary to switch to paid stock sites that control their sources to avoid this risk?
(Disclaimer: I am asking out of curiosity; and because I would like to see this kind of question on this site. Whether this is in graphicDesign.SE's scope or not, the community will have to decide!)