It appears confirmation of Left 4 Dead 3 has surfaced on account of someone at Valve forgetting to turn off a monitor.
IGN reports on an image taken by an unidentified Dota 2 player taking a tour of Valve's offices as part of The International 3 tournament.
The shot lists the unannounced third title in the developer's cooperative-based zombie shooter franchise, along with the previously confirmed Source 2 engine, under current projects.
It makes sense that Valve would continue to build on the success the L4D series has seen with a totally new effort – it's just the first mention anyone has actually been able to pry from the company.
The report remains unconfirmed for now, as Valve won't comment on the report and likely will only announce L4D3 when it's good and ready.