Quality is delighting customers
As Amira pointed out, you could either report the bug in the bug db & then add this test case to your test case repository.
Or, if in case, you make any references of your test case id in the bug that you log, first add the test case & then log the bug.
Test condition is present in the MRD(requirement document) but not included in the test case in this case update the test condition for the respective test case and track the issue by providing the steps to reproduce.
Test condition is not present in the MRD but that functionality is required at the customer end point of view in this case track the issue as enhancement bug.
Thats not a critical sitution ,its very common for testers...many times testers first prepare testcases then excute them but not found any bug ...then they try to do some monkey testing and found some bug ...then for reporting this bug we should write steps to reproduce and prepare testcase for same ...:)