Simple mistakes such as,
1. Providing inconsistent data
Some people have different names on the certificates, others have different date of births etc
2. Failure to provide correct data when filling the online portal. Some people just complete the online portal without cross checking to see if their telephone numbers, email address and postal address are all correct.
3. When people with better grades than yours apply and the school capacity is low;
Point 3 is the only issue you should be worried about if you are sure that you made no mistake whatsoever