Valve have updated the Steam trading process with a new CAPTCHA box that pops up before the trade goes through, to confirm that you're not a robot. Are you a robot? ARE YOU? It's just so hard to tell, and I don't think ticking a box is a good equivalent to Blade Runner's Voight-Kampff machine. It does seem to be a pretty good way to stop malware from making trades on your behalf, however, which is why Valve have implemented the damn thing in the first place. See, here they are saying exactly that.
To save you the trouble of clicking that link with your jagged robot hands, I'll copy and paste the key bits.
"We’re updating trading to include a captcha as part of confirmation process. This is to prevent malware on users’ machines making trades on their behalf. We know it’s a bit of a hassle, and we don’t like making trading harder for users, but we do expect it to significantly help customers who are tricked into downloading and running malware from losing their items."
Valve have excluded a few third-party trading services from the CAPTCHA process, so if you use any of those, you might not have noticed any changes at all. Since the announcement and implementation, Valve have also swapped the traditional 'type a load of nonsense' CAPTCHA box for that new styled one that accepts a simple box tick instead. I have no idea how that works, but I assume it's magic. I've cropped the above image from the SteamDB tweet where I heard that, and here's where the news originally came from.
Some traders have been experiencing a few problems with the new trading process, including existing trade offers being reset.