North Korea fired 4 missiles during US-SK joint military exercise.

http://www.reuters.com/article/us-northkorea-missiles-idUSKBN16C0YU

http://www.bbc.com/news/world-asia-39175704

The US and South Korea started its massive joint military exercise involving 300,000 personnel and latest radars such as X-band and THAAD.  However, North Korea fired 4 missiles during the joint exercise as a sign of protest.  The NK missiles revealed critical flaws in the radars as they were not able to detect the types of NK missiles.  This put to questions about the effectiveness of THAAD defence system because it couldn't detect the types of missiles fired.

When you cannot detect the types of missiles, you cannot intercept them effectively especially for multiple-head missiles because the multiple-head missiles can contain real and dummy warheads.