1. While testing a s/w which has authentication, we can do negative testing by entering wrong credentials.
2. Suppose the requirement says that a text box can accept ony alphabets.
Negative test case will be : Try entering Numbers and special characters.
There can be no. of examples depending on the type of s/w you are testing.
Read this out for more clarification on negative testing
Negative Testing = (Showing error when not supposed to) + (Not showing error when supposed to)(Usually these situations crop up during boundary testing or cause-effect testing.) Here if either of the situations in parentheses happens you have a negative test in terms of its result - again, not what the test was hoping to find. The application did what it was not supposed to do. User tends to put negative values, which may crash the application.
In case of Cellphone, Some examples for negative testing are:
1. Receive Incoming call, during that press green key again again , See what happen.
2. Full Inbox/sentbox of message/email/MMS and again send the message on that device, or save the messages in sentbox.
3. Remove the Sim card from device try to do network (GRPS / CDMA) related activities, watch the response.
4. Dial call or receive call during device battery is empty.
5. If communication is going on device, remove the battery of device and again attache battery to device, see what happen.
6. If communication is going on device, attached the charger to the device.
I think these are the negative test cases for cellphone.