1. While doing execution, unexpectedly some of the 
applications are down or environment unavailability.

2. Data issues i.e.,proper data has not been provided for 
executing the test case.

3. Some of the application links are not working which are 
provided in environment sheet.

see it is so simple:
during the project past what was the most difficult 
part,its understanding the SRS  writting testcases
for example in ur project there is one application which is 
retriving data from some database table..how will u write 
testcases for those modules  which keep changing,this was 
just an example..
every project has some easy and difficult part and see if u 
are sinscere that difficult part after completing seems to 
be easy.. and u will be clear about it.
so always focus on tha difficult module which you solved it 
in that way they will ubderstand that u really work on the 
project and took pains in delivering the right part.

On my point i am telling you....
I faced a lot in writing testcases for reports.Our 
application has reports with multiple selections at a 
time,means it has to consider the parameters mentioned and 
show the report according to the selecting parameters.On 
the fly we have to select the parameters.so for that we hav 
made seperate set of testcase.....
I hope like this you can say....