I experienced a strange problem with my website which used to work correctly under 3.8.6.
I'll try to give you all the details I can find out about it.
I started from 3.8.6, upgraded the whole thing following the tutorial on ez.no. I used the dbupdate script removing the 3.8.1 and 3.8.5 parts, I converted the tables in innodb, and executed every update script which didn't report any error.
The website works, exactly untile the cache expires automatically. Then, after that, it will remain with a wait page loading forever.
I tried to grow up the execution time of scripts and php memory (3600 seconds, and 256mb of ram), disabled smartcache clearing system, development mode is disabled. If I disable template compilation the website works again. The templates make heavy use of cache blocks.
After the cache probably expired, when I did my page reload (which hanged in forever loading state), I found these two rows in error.log:

[ Apr 13 2007 13:48:12 ] [xxx.xxx.xxx.xxx] eZTemplate:Attribute node-object-cached of variable $resourceData was not found but variable node EZ_TEMPLATE_TYPE_OPTIMIZED_NODE is still present. This should not happen:

[ Apr 13 2007 13:48:12 ] [xxx.xxx.xxx.xxx] eZTemplate:Attribute node-object-cached of variable $resourceData was not found but variable node EZ_TEMPLATE_TYPE_OPTIMIZED_NODE is still present. This should not happen:

I have the strong suspect it could be a bug. If u need more help I'm available to collaborate, and give you access to this development server in case you want to see some config files or whatever.

Steps to reproduce

Update a 3.8.6 website with heavy templates, wait for the cache to expire and reload the page. Hard to reproduce tho.