Calculation precision
Logical errors,
Performance related(becoz it must be C/S,Web enabled system)
Database errors.
secutiry...

if it is online the following bugs
are
1.UI defects
2.Usability
3.Online
4.security
5.server problems
6.Database
 
If you still have any doubt feel free to mail me at :  
rajendra_penumalli@yahoo.com
or call me:9885162742