Anyone got this error message before? "Your account credentials have expired. Please re-enter your login details (or something like that)"? I just got it for the first time today. Before this, my Steam was having trouble connecting to the network, so my Friends list was inaccessible. I could still download stuff from the games library though.
Not sure if it's my account being hijacked or anything like that. I had no problems re-logging in though. On the Steam forums, a few newbs claim it is account being hijacked, while there are also people who say it is just an ordinary message. Has anyone had this message before?

Sure, that's happened to me a couple times, usually when there's been some major update to Steam itself or some of my Valve games. I don't think it's anything to worry about; I was able to log in again just fine and didn't detect any irregularities.

Is this through the steam app or the website? Pick whichever one it isn't and go & change your password just to be safe, its good practice to change it occasionally anyway

This might be a glitch with offline mode; as far as I know offline mode only works for a certain period, after which you need to re-authenticate online, so it's possible that it somehow thinks you've been offline.
Having said that, yeah, change your password just to be safe.

The former. I'll probably change my pw soon, but just want to make sure on this note first. Have both you and Arkose ever gotten this error before?

I don't remember getting it, I know I've had to log in again after an extended period of not using it but I don't think it was that message.
There was some mention of a database crash the other day that had some purchases vanish until they restored the system, might be connected to that

I haven't had this particular error, but I use Steam very rarely so the likelihood of me seeing an already-rare error message is very low.

A few times this week I have gotten an error that I could not connect to the network. Some other people in their forums posted the same problem. So maybe the credentials expired and since it could not log in you got the error.

I have never gotten that message either

How long is that period? I used STEAM in offline mode for several months without issues.
I think I got that message once or twice, I think it was after an update.
But just to be sure:
Did you let your E-Mail be verified? Under Settings > Verify E-Mail Address... you can make STEAM verify your Mail. You will then get an E-Mail in your postbox which you need to confirm! After that any attempts to change your password/E-Mail address will have to be confirmed by Mail.
But maybe you already did that check ;)

I think there is nothing to worry about. It happened to me few times when I left the PC untouched for a day or so with Steam running. Also, several months ago, when I was trying to launch L4D2, it was bothering me with this very issue. It could be related to the problems the servers are having right now. Yesterday, I was not able to play my games for one hour or so.

Never seen it. The only error message I got of late re:steam was when buying something last week. For donkey's I've been able to buy games with an unverified email address. This time they insisted I let them verify the address before letting me purchase anything.

It's happened to me before. It was after five hours of not using the Steam client, so I figured it was telling me it logged me out because many services log you out after a set amount of time.

Yep I have already verified my email. Didn't get any emails from STEAM telling me that my password has been reset or anything like that.
Thanks to all who also had encountered this issue. Good to know that it's probably not a big deal. I suppose it was the server's fault after all, as I was having a hard time connecting to the network (Friends list) when this happened too.