Combo killer, a new achievement that was introduced with Jan 28’s daily achievements, has been removed from the weekly rotation due to its buggy nature.
The combo killer has been removed from the weekly rotation, clearly not working as intended.
Colin Johanson clarifies a bit more about the daily changes.
This system for picking from a list for dailies, along with rotating in new more specific achievements will be added in a later release to build on the system going forward. As discussed in the Gw2 in 2013 blog post this is really just the ground work for the systems coming down the road.