URL contains upper case characters
This means that the URL in question contains upper case characters.
Why is this important?
Mixed case URLs can cause issues when both the lower case version of the URL
(e.g. https://example.com/page) and the mixed case version (e.g. https://example.com/Page) respond with a 200 status code. It is often also the case that other variations will also be accessible
What this leads to is multiple pages with exact duplicate content, and depending on the scale of the issue, this could trigger 'quality' algorithms for search engines (e.g. Google's Panda algorithm). If it is a problem at scale, this may also cause issues with crawl budget, as search engines may need to crawl thousands of extra pages unnecessarily.
The existence of multiple pages with the same content also leads to other issues, such as the understanding of which one is the canonical version, and inbound links potentially pointing at the 'wrong' version.
What does the Hint check?
This Hint will trigger for any internal URL which contains at least one uppercase character.
Examples that trigger this Hint
The Hint will trigger for any URL with upper cases characters in any part of the path:
Why is this Hint marked 'Potential Issue'?
This Hint is a 'Potential Issue', which means that it may not be affecting the site at the moment, but should be investigated as it could cause issues in the future.
Ideally, URLs should always be lower case. There are some instances where this is less important, such as URLs that represent downloadable files (e.g. PDFs or Word Docs) and the filename itself is upper case. Typically in these cases, there is only one version of the file, and the understanding of the canonical is unambiguous.
For HTML URLs, however, it makes sense to stick to lower case. If mixed case URLs exist, you will need to dig into the issue further to understand the scale of the issue. If lots of mixed case URLs exist, and their lower case equivalents are also being crawled, you may have a lot more cleanup to do - if, for example, Panda has been tripped.
If it is just a few URLs that are affected, it should be a lot easier to resolve. Go through the incoming links to each URL, go onto each of these pages, and edit the href values so that they link to the lower case version.
Once this is done, recrawl the site to ensure that all the mixed case URLs are gone. You may also wish to do a permanent 301 redirect of uppercase to lowercase, in which case we recommend you check out this redirect generator from Aleyda Solis.