I accidentally pasted (L)GPL / CC-SA snippet my code, what do I do now?(news.ycombinator.com)

https://news.ycombinator.com/item?id=2137389 What would happen if Microsoft or Google were to have a small snippet from stackoverflow / GNU software and it was found it was in their for years? How many programmers here ever have copied directly from Stackoverflow or L(GPL) code? If you were to, and you removed the accidental viral copying - would you still have to release the code?