Task become milestone automatically when calendar is changed

Hi,

In MS Project 2003 project, there is a task,say task1 that has been marked
done i.e % complete and % work complete is 100 and is assigned a resource say
r1.In this project if I goto Tools >> Change working time and change timing
of a resource calendar other than r1,then the duration of task1 becomes 0 and
task becomes a milestone.Why is this happening?How are they related?

Other thing is that it happens only on MS Project that has no service
pack.On machines with SP2 or SP3 for MS Project this issue does not occur.So
cannot understand if something is wrong with my project or is it a Project
bug..

Vivek Gupta,
MCTS,MS Project 2007
.