I was sure that ths is an error wich newer come back to me - was a relatively common one in early 2010-s, since dissappeared from my radar. Today one of my former students approached me with his problem. He took over a site running the latest Joomla, and when tried to change the blog list views to category blog views, as the siteowner requested, the respective pages ended up with a white PHP fatal error death screen.
Asked him to turn on debug and send me what's on the screen, and yeah, an old problem showed up:
Fatal error: Call to undefined method JHtml::core() in ... /html/com_content/category/blog_item.php
Yeah, his site used the blog template HTML override from some ancient template, "featuring" a long time deprecated Joomla API call.
The solution to the problem is simple, like a clean lightsaber cut: comment out the troublesome line, and replace it with a correct call oin the blog_item.php: