A few days ago two cargo ships collided in the North Sea in the beginning or just before the TE Traffic Separation Scheme, the 395 m (1200 ft) long CMA CGM Laperouse and the 90 m (300 ft) Thebe, luckily with little damage. Conditions were not great, 3m swell with a 40 - 45 knot wind, but not completely atrocious either.
You can see where the accident happened on MarineTraffic's AIS log of the Laperouse's track.
Here is a scanned recording of the VHF transmissions of the the Dutch Coastguard and rescue services (from scannernet.nl).
Now both of these vessels are mandated to have AIS on board, but that doesn't say both had fully integrated AIS sets that would warn them about an upcoming collision. Even so, it's hard to understand why these accidents still happen even with all this tech aboard.
It shows that although AIS is a great help to all parties there is apparently still enough room for error such that a large(ish) cargo vessel is able to collide with one of the largest container vessels in the world.