Just wanna point this out in case nobody else has yet.
If you receive an e-mail notification of a PM or reply to a topic, if you set it so that the e-mail will show you what the PM/topic message says, and if the PM/post has spoiler tags in them, the e-mail message will show these spoilers without being hidden.
Not sure how much I can do about it (other than clearly warn people), but this is definitely something that should be looked into. I probably won't get a chance today or tomorrow, but I'll try to remember to take a look sometime on Sunday. (I'll probably look at the sub-sub-forums not being included in latest post check (http://www.lemmingsforums.net/index.php?topic=2656.0) at the same time; may as well address both while working on the site.)
At the very least, I should be able to remove the spoiler-tagged text from the email altogether, and add a notice saying something along the lines of "This message contains spoiler tags which have been removed here, please view on the forums if you want to see the content of these."