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Yep; that was it. I just analyzed the page for any crawl issues, and the 4xx error didn't come up as one.

I think that I figured it out. Once I did more research and learned that a “<” appears as “%3C”, when you add the “/div” after it, it clearly had something with improperly closely a div. After scanning my page source, I found that I actually forgot to finish a link, so it was “[”. I hope this fixes it.](</div)

I suddenly have 4xx errors in my crawl diagnostics because pages with “/%3C/div” added to the end of the URL that are linked from the normal page can't be found. I didn't create the pages, and they don't exist, but Moz thinks that they do. I went back through to see if any changes in WordPress, theme or plugins versions might be the cause, but this is the only site that I have this issue, so I don't think that is it. Does anyone have an idea what causes this?

Yep; that was it. I just analyzed the page for any crawl issues, and the 4xx error didn't come up as one.