I had two shocks over the past two weeks that were unexpected - in each instance, a Wordpress plugin was causing serious display problems on two of our financial sites.
The first was on Finance Markets, which covers personal financial news.
The related posts plugin being used on the site suddenly stopped working properly. The result was that it now took an age for the pages to load up in IE, because it was waiting for the related posts plugin to stop searching and finally display the related posts intended.
I tried an upgrade, but found it didn't solve the problem - the whole process was just taking far too long.
In the end, I found actually downgrading to a simpler related posts plugin allowed the site to start running at proper speed again.
The other instance was with Banking Times, dealing with news in the banking sector.
Again, it was an errant plugin, this time the supercache plugin. Even though it was no longer supposed to be active, it no longer allowed URLs to be displayed correctly.
It actually took a long time to figure out what was happening - I kept editing the htaccess file, and found the site worked when blank - which is bad because all of the URLs are rewritten. That was no solution, as the site would have been completely restructured.
It was only after a lot of frsutrated tweaking and exploring I eventually found an error message I could trace the cause of. Even still, getting rid of supercache wasn't as easy as I'd thought, and ended up having to use phpmyadmin to remove all trace of the plugin directly from the database itself.
Once done, the site started working fine again.
It both instances, though, a lot of unnecessary frustration.
I normally take care with plugins, and try to use the minimum required. That's still a good philosophy I think.