A misdiagnosis could be welcome or unwelcome, depending on what was misdiagnosed, but there is one thing to keep in mind regardless: Your health as a patient was at risk. If you were improperly diagnosed as being healthy, you wouldn't be receiving the care you need. Likewise, if you were improperly diagnosed with a disease, you might receive treatments that have side effects you didn't need to be exposed to.
There are reasons for misdiagnoses, which primarily come down to human error or errors in testing. When a doctor begins examining a patient, they look into a differential diagnosis. This is a process of looking at all the symptoms that line up with multiple illnesses and then slowly eliminating each diagnosis one by one. Eliminating possible diagnoses is only possible through further testing or observation in most cases.
The problem occurs when a doctor doesn't perform this step accurately. Eliminating one possible diagnosis before even considering it is one possible issue. Another would be sending out for test results and getting the wrong patient's information back or being informed incorrectly about the results of the test. Any of these things can lead to a patient with an incorrect diagnosis.
Once a patient is misdiagnosed, there is a risk that they will continue to receive, or not receive, care appropriate for the condition. For a patient without cancer, it could mean getting an unnecessary round of chemotherapy. For someone with it, it could mean the cancer spreading due to not receiving it.
Misdiagnosing a patient is extremely serious, and it's something that you should address with your medical provider immediately. Not all cases of misdiagnoses come down to malpractice, but many do.