Google's Senior VP of Engineering Vic Gundotra was asked not to tweet again after he posted a tweet which critiqued Nokia's deal with Microsoft.
Gundotra revealed this himself in a recent interview, which you can see in the video above (the interesting part is at 1:01:37). Responding to a question why he has stopped tweeting, Gundotra said "I made one tweet, and then I was asked not to tweet again (...) by my boss".
The offending tweet was posted right after the Nokia-Microsoft deal was publicized in February 2011. It said "Two turkeys do not make an Eagle," quite obviously referencing the Finnish mobile giant and Microsoft's Windows Phone platform which Nokia had embraced.
After that, Gundotra tweeted only four times, last time in July 2011.
#feb11 "Two turkeys do not make an Eagle".— vicgundotra (@vicgundotra) February 9, 2011
By Gundotra's own admission, he made an error in judgment very common in the Twitter era -- he thought he was talking to a smart number of people, but his tweet went viral.