I used to get a warning when completing a lesson would automatically mark remaining blue words as known, but I just discovered that I no longer get warned (and thus accidentally marked a lot of blue words as known).
Is it possible to turn this warning back on?
there were already a lot of complains about this problem. But they refuse to correct it or even admit it is a problem.
Unfortunately, I think the warning only occurs the first time you complete a lesson (?) 
No way to currently turn it “back on” or set it as a permanent fixture.
Rooster Master LingQ as an alert for dangerous buttons like these, to help prevent this from happening.
Wish LingQ would also include an easy feature like this.
I honestly wouldn’t like to have another click to do at the end of each lesson. If there is the possibility to add the option to have the warning, or not, it would be better, because I definitely prefer less clicking.
They could make it on by default for users that turn the “paging moves words to known” feature off. Or even create a toggle that allows you to have warnings for clicking the end of the course button on, but those suggestions may be too complex for LingQ to implement (b/c they seem to be having trouble implementing a basic warning for users, period), but many users accidentally clicking —because of the interface design — finish course versus more clicking is a much more destructive action in LingQ currently.
So I think having a warming message pop up for users is a far better option rather than not implementing it because it adds some extra clicking.
I agree that the problem is mostly for people that have “paging moves words to known” disabled. But if you turn that feature off, I suppose you have a logic in your studies that might made you aware of it. I see people that mention this problem especially at the beginning, I probably made that mistake as well a couple of times. I guess we learn more by suffering. 