It is a type of informal and random free style testing that is conducted on
an application without using the test case and adhoc testing is done to to
cover the uncovered areas in the test case documents. Disadvantages of Adhoc
testing

Unorganized testing

possibility of not covering of major functionality

can t assume the time to work on the application

cant show the proof that so and so of work has been covered cover

sometimes cant reproduce the bugs which I found.

With the help of a prepared Test case we can give some input data and
verifying the expected results here we can analyze the application blindly by
following the Test cases but it s not possible with adhoc testing as you need
to keep a track of what kind of input has been given and based on that expected
should be analyzed and on executing you need to conclude whether that particular
adhoc test is passed or failed to do these things without following predefined
test cases you need spend lots of time and should be more cautious.

Ad-hoc testing supplements structured functional black-box testing and is not meant to stand on its own. Typically testers are asked to use their imaginations to find defects outside their structured tests. The drawbacks with ad-hoc testing are

1) Test execution is not organized and testers can pursue identical paths thereby making the process inefficient
2) Tests are not traceable and after all is said and done it really isn't known what has been tested outside the defects that have been logged
3) There is no means to ensure tester productivity during these exercises and
4) Crucial functional areas of the application may get overlooked.

How do you overcome the drawbacks of ad-hoc testing? You do this by ensuring that your structured functional tests have been exhaustively completed before you move on to ad-hoc testing.