Diffect arises due to following things & there result in 
test plan are as follows :-

1-due Insufficient resourses failure of scedule for 
intended activity,

2-due to changing exit criteria failure of test plan,

3-unskill tester so it become hard to assign work to proper 
person & they can not properly recognise features to be 
tested. 

For Futher disscussion please 
contact:prabha.koshti@gmail.com

Mainly defect is occurred because missing extra or wrong 

when we get the requirement doc. and these three entities
are present then at the time of preparing the test plan we
will get the defect