Not a doctor but this happened to me.

So I had been having difficulty breathing and this gradually got worse and worse to the point where if I laid down horizontal I wouldn't be able to breathe properly. Every night I had to increase the angle at which I slept with pillows just to get some shallow breathing going.
Well what I initially thought was the flu was quickly becoming something I had never felt before. It was a Sunday so we couldn't visit my regular GP but Dad managed to find one place that was open.
I go in and the doctor sees me, she does all the usual stethoscope stuff and sends me into the next room for an EKG. Results don't show anything so the doctor says it looks like the flu and to take Paracetamol.

The next day was worse and I didn't really sleep so first thing in the morning my Dad takes me to my regular GP and as soon as that cold Stethoscope touched my skin the word "Pneumonia" left my doctor's mouth. He said it sounds bad and I need to go to the ER straight away. So we decline the ambulance and Dad drives me over, at this point still not realising exactly how bad of a state my left lung is in but shocked at how fluid in my lungs can be misdiagnosed as the flu.

They admit me, get me on IV antibiotics and I'm hoping a couple days of this and it will be fine. Until later that day when the doctor assigned to me comes in and tells me he's gonna need to stick a huge syringe, I think it was referred to as a tap, into my lung from the back and draw it all out. In fact I was told how lucky I was, if it had taken any longer for me to come in they would have had to cut me open. As the doctors here know, pneumonia starts off as liquid but your body will begin to wall it off in order to fight it. From what I understand this means complications.

I had never had any sort of serious operations or surgeries so I was surprised by the abruptness the next day when the doctor came in ready to go, I was already in a private room so he said I could go ahead and take my shirt off and we can get started.
"Oh and by the way, these 6 students are gonna observe."
He pulled 1.7Litres of this stuff out of my left lung, which caused it to collapse requiring about a week's stay including some minor rehab. Breathing exercises and getting me to walk around.

My GP is probably one of the most professional doctors I know. I have seen some doctors who seem very casual and don't inspire much confidence.

In fact I had a similar thing happen again; I had what I thought was tonsillitis based on prior experience, and again we find ourselves on a Sunday afternoon, so we looked for another after hours clinic intending to avoid what happened last time. This time the diagnosis was correct but she prescribed me some weaker antibiotics, 3 to be exact. I assumed this is for early tonsillitis.
But experience dictates I should get a secondary (primary?) opinion from my regular GP at which point he prescribed legitimate phenoxymethylpenicillin.

So as a cautionary tale to all those who self-diagnose; If a doctor can get it wrong so can you.

And find someone you can trust!