Just for the record, I've tried the usuals: CTRL F5 for full refresh, spawning a new browser with no other tabs, reboot and new browser. This has been an issue since yesterday for me.
Ah, we did have the odd glitch the other day - like all of the servers going down at our hosting facility! - so it's likely once the error happened, your browser was just reloading the error page from cache.
Apologies folks, we did have some unwanted Error messages yesterday but the site seems to be fine today. If it does happen again, clear your cache and quit then restart your browser as Andrew suggested. That usually does the trick.