While reading about Doctrine's cache mechanism (which applies to other stuff than database queries, by the way), my eye was caught by a little message at the end (last section) about cache slams.
I have used cache mechanisms extensively over the last few years, but (maybe luckily) never happened to witness a "cache slam".
There's a link to a blog (by an unnamed author) that explains that.
To make it short, you can have race conditions in APC (and probably in other caching mechanisms in PHP) when you assign a specific time for expiry of cache data, and a user gets to that expiry time at the same time (or very very very closely) as other users. This provokes a chain reaction (a little bit like an atomic bomb, but not with the same effect - unless some crazy military scientist binds a high-traffic website to the initiation process of an atomic bomb) which makes your website eat all memory and freeze (or something like that).
In reply to me mentioning it on Twitter, @PierreJoye (from the PHP development team) kindly pointed me to APCu, which is a user-land caching mechanism (or so to speak an APC without the opcode, and simplified).
Apparently, this one doesn't have the cache slam issue (although I haven't checked it myself, I have faith in Pierre here) and it's already in PECL (still in beta though), so if you want to try it out on Debian/Ubuntu, you will probably be able to sort it out with a simple:
sudo apt-get install php5-dev php5-pear make
sudo pecl install APCu
(and then configure your PHP to include it).
Don't forget that it is a PECL library, meaning it's most likely you'll have to recompile your PHP to enable it, but PECL should handle that just fine (in our case it's a bit more complicated if we want to avoid asking our users - of Chamilo, that is - for more dependencies).
Anyway, just so you know, there are people like that who like to make the world a better place for us, PHP developers, so that we can make the world a better place developing professional-grade, super-efficient free software in PHP! Don't miss out on contributing to that!