As a patient I had a doctor tell me "I don't think this is important".
My physio had identified a mole on my back that had changed colours. Being fair skinned, and Australian, I was obviously concerned about melanoma so I went to my doctor for a checkup. He looked at it and said "it's nothing don't worry". I insisted that since I'd made the effort to come in he might as well take a biopsy and get it tested. He was reluctant but I insisted and he eventually removed the mole for testing.
A few days later the doctor rang and informed me that I had a very nasty malignant melanoma and needed surgery immediately if not before. Two days later I was in hospital and they removed a lump from my shoulder three inches across and two inches deep. Cancer free now for 8 years but if I had taken the doctor at his word of "it's nothing" I'd most likely be dead now.