Long before Geoffrey Hinton and Eliezer Yudkowsky sounded warnings about artificial intelligence, there was Dr. Norbert Wiener in the 1950s and 60s. Wiener was a pioneer in the field of Artificial Intelligence, or as he called it ‘Cybernetics.’ His groundbreaking work laid the foundations that our current AI boom is built atop of and his concerns about AI have no doubt influenced those being voiced by his successors in the field today.
People have foreseen and written about the consequences of human-level and superintelligent AI for a long time, which if anything suggests that such concerns are grounded. Weiner & others of his era underestimated the complexity of the task of creating AGI. We've never actually been close to it before in history. But we are now.
Weiner vs Wiener
If you see the capabilities of humans as basically constant, and those of machines as going up. It makes sense to think that machines will probably one day be far more powerful than humans.
As these people didn't give a date on their warnings, it's hard to tell if they were wrong, or predicted something that is still in our future.