What can cause a 410 error on a live url?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  George G.: Check your http headers.
  
  Do you have a seo plugin that might include the 410 in the http headers? Check that too.
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  Neil Cheesman: Yoast - I need to find out where Yoast `lists` 410 urls... assuming they do - or if they get added to htaccess or somewhere. I think given a 410 in error... during some recent revamping
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  George G.: Neil Cheesman see the http headers first
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  Neil Cheesman: The header is fine - it was deleted and set as a 410 in Yoast - when it shouldn`t have been...
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  Neil Cheesman: The url was removed and given a 410 in error a couple of days ago - via Yoast - now when trying to submit the url via GSC it won`t reindex as GSC says 404 not found - BUT it is a live url.
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  George G.: paste the url
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  Neil Cheesman: https://www.londontheatre1.com/.../shrek-the-musical-at.../
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  Neil Cheesman: I am sure I am missing something but can`t see the wood for the trees..
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  George G.: Neil I told you so, your WP inserts 410 in the header. You need to see on your backend, yoast or some other plugin insert 410 in the page. Also it is a good idea to upgrade to PHP7
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  Neil Cheesman: Where are you seeing this? (not in the page source)
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  George G.: i use curl in a terminal.
  
  you can check it in any online tool like this one https://securityheaders.com/...
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  Neil Cheesman: Thanks - I have been looking but not sure where to find/edit the header etc... I checked out Yoast online and they simply say re-crawl etc... in GSC - but clearly that doesn`t work..
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  Richard Hearne: It`s a HTTP header. Not in the source, but part of the network traffic related to the page. You cannot depend on what you see rendered int he page to determine the header. CURL is a bit of a monster, but basic usage is well worth learning. Even more so if you`re into Technical SEO in any way.
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  Dave Elliott: Theres loads of browser plugins that will give you header response info as well. Much easier.
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  Neil Cheesman: FIXED IT - simply need to put the url (a product) back into drafts and re-publish... D`oh!
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  Richard Hearne: Be careful. That does not sound like regular WP behavior. Publishing the page may have updated something that changed the header settings, but that doesn`t mean that the cause of this has gone away. Keep an eye on it for a wee bit.
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  George G.: Richard Hearne its probably a yoast bug that happens in some extreme cases so nobody has paid attention to it.
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  Neil Cheesman: I saw that the url wasn`t in the sitemap... put into draft and then re-published and it appeared. It was an unusual scenario and basically down to humann error that a handfull of urls were given 410s but then realised they shouldn`t have been.
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  Richard Hearne: George G. Possibly, but it would be a very destructive bug. Not something to leave unpatched.
  
  If it is from Yoast I think it might be from the bug where they allowed attachment URLs to get indexed, and then brought out the fix which was to 410 those URLs and submit a new sitemap to get them crawled and deindexed.
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  George G.: Richard Hearne yea yoast got history of having quite a few bugs. also yes, could be related.