[|Erasmus|]

Just as an excercise, the spoiler tags on this website are implemented as a link with the href set to "#" which is shorthand for the current page, and if you just have a link to # will cause you to scroll to the top of the page.

There is also an onclick written in javascript which hides/shows a bunch of text (hence the spoiler bit).

Given that you get shifted to the top of the page, and the spoiler fails to show, I can only assume that this occurs when something weird happens and the page fails to load fully (i.e. if the script that runs on the spoiler isn't downloaded correctly it's just a plain old link to "#").

Since I've only ever seen this happen at work where I'm behind a proxy which behaves strangely and occasionally causes firefox to just hang for a few seconds, it's likely due to something similar happening for you and causing the page to get interrupted while it's loading...