I have observed the same behavior using version 1.5.1 RC1.1 on 64-bit RHEL 5.3 and Adobe Air 1.5.1 release 8210. When set to Snapping constrains to 15 minute increments, entries dragged into the Week view from the task tree will snap to 11:59, 12:14, 12:29, etc.
The most unusual part is that after Klok is open for “a while” it snaps according to my snap preference on the hour, for example to 12:00, 12:15, 12:30, etc. I can’t duplicate how to make it snap to these on-the-hour times, yet, it just starts working suddenly after it’s been open for a while. I haven’t been able to figure out if it changes behavior after being open for a certain amount of time, or if there is some action I unknowingly take that eventually gets it working the way I expect.
What I can duplicate locally is that, by restarting Klok, entries dragged from the task tree to the Week view snap to 1 minute off the hour times.