Need hints/clues to a mind f_ck. Ideas?

I've got a plan to DM everyone through the normal fantasy genre that is D&D. But when they reach max level, beat the boss, or somehow experience TPK, instead of dying they wake up to realize they were in virtual reality the whole time.

Full dive with memory alteration to make the 'game' more enthralling. I'll either end the campaign there or make plans into a more sci-fi themed campaign. But for now I' want to try this.

What could be some hints or clues through out the fantasy campaign that might make the players think something isn't right? Besides rendering issues or minor glitches.