The NY Times published a piece on Sunday about how Pakistan likely showed Chinese engineers the pieces of the American Black Hawk helicopter that crashed during the Osama Bin Laden raid. This helicopter is special because it had classified stealth technology.

From the Times:
American officials cautioned that they did not yet have definitive proof that the Chinese were allowed to visit to Abbottabad. They said that Pakistani officials had denied that they showed the advanced helicopter technology to other foreign governments. One military official said Sunday that Pakistani officials had been directly confronted about the American intelligence.
One person with knowledge of the intelligence assessments said that the American case was based mostly on intercepted conversations in which Pakistani officials discussed inviting the Chinese to the crash site. He characterized intelligence officials as being “certain” that Chinese engineers were able to photograph the helicopter and even walk away with samples of the wreckage. The tail has been shipped back to the United States, according to American officials.