A number of users on Samsung's forums and XDA have been reporting issues with some Qi chargers, including those manufactured by Samsung. Some devices aren't able to make use of Qi's Fast Charge feature, and in some cases they are failing to charge at all. The chargers made by Samsung specifically for the S8 and S8+ seem to be working fine, but it's strange that other Qi certified chargers, including Samsung's older ones, aren't having the same success.
Affected devices allegedly manifest the issue through a repeated "Wireless Charging Paused" toast notification and a later notification of "Irregular charging detected." Samsung's most recent version of the charger appears to be unaffected, but reports include a pretty wide variety of other brands and other models of Samsung Qi chargers. For some it only seems to affect fast-charging, and the slower variety works fine. An excellent video demonstrating the issue was put up by one user:
At this point there isn't a lot of information as to exactly why or how one might run into the problem, but it seems to be pretty widespread. There are inconsistencies in reports for different combinations of bricks, cables, and Qi chargers that supposedly do or don't cause it, and our own configuration like that in the video above demonstrated no similar problems. If you find yourself suffering from this, some have reported that the issue goes away for a period after a reboot. Either way, it might be worth checking your phone for the warning after you toss it on your Qi charger before bed.
We've reached out to Samsung for comment and will update this post with any new information.