how to test buggy application

The best bet in this situation is 
for the testers to go through the process 
of reporting whatever bugs or blocking-type 
problems initially show up, with the focus
being on critical bugs. Since this type of
problem can severely affect schedules, 
and indicates deeper problems in the
software development process
(such as insufficient unit testing or
insufficient integration testing, poor
design, improper build or release procedures, etc.) 
managers should be notified, and provided
with some documentation as evidence of the problem.