GameInformer reports that manual saves were vanishing, and that older saves were wiped of some data, notably side quests. PlayStation 4, Xbox One, and PC versions were said to be affected, and those running on Windows 7 were able to fix the game by setting the clock to April 8.
There have also been numerous forum posts by affected PC users on the Thief Steam forum complaining about their saves being corrupted.
Square Enix confirmed that it was aware of the issue. "We're aware of an issue that seems to be affecting some Thief players, preventing them from completing some levels,” said a company representative in a statement.
The error is highly unlikely to be an intentional April Fool's gag on the part of publisher Square Enix or Eidos Montreal.
“We're taking this very seriously--as a first step we're reading as many reports of the issue as possible, working to replicate it internally, so that we can understand what's causing it. We're sorry for any disruption to your game today, and we'll update you as soon as we have more information," concluded Square Enix's official response.
GameSpot has contacted Square Enix for more information.