Apple CEO Tim Cook tweeted a picture of his latest visit to the Mac Pro assembly plant in Austin, Texas, only for users to discover a tiny detail in the image that showed a Mac running on Windows OS.

Apple CEO Tim Cook's inaugural speech at the WWDC 2014 this week began with a jab at Microsoft and the sluggish adoption rate of its latest OS, Windows 8. But less than a week after his mockery of the  Windows OS, Cook's tweeted a picture of his recent visit to the Mac Pro assembly plant & AppleCare center in Austin, Texas. The irony here is the tiny detail in the pic, which Cook probably didn't notice but others sure did. The sharp eyed folks at Business Insider spotted the Macs used in the Apple production facility had a version of Windows running on them.

Cook was accompanied by the company's Senior VP of Internet Software and Services Eddy Cue during the visit to the Austin campus, Thursday. Cook shared his excitement during the tour with the millions of followers on Twitter just the next day. But only had he known how a shared picture could go viral in a matter of hours.

"It's not the first time we've gotten a look behind the scenes at an Apple factory with Macs running Windows. Foxconn, the Chinese manufacturer that makes most of Apple's gadgets, uses Windows machines too," Business Insider reporter Maya Kosoff wrote.

Several users also managed to spot the Windows running on Macs in the background. Since Cook has merely 64 tweets to his name, every single tweet matters on the social network. The conflicting pic shared by Cook was retweeted thousands of times with comments from followers about the Macs running Windows.

The picture does not clearly show what version of Windows is being run on the Macs but most comments vote in favor of Windows 7 or Windows XP. According to PC World, Windows on Mac was possibly used to run electronic design automation software or some other program.

We have snapped a screenshot of the comments from Cook's followers about the use of Windows OS on a Mac computer.