This wasn't a mistake but fits with the kind of story we are sharing
Background 1
I was what was then called a 'Cardiac Technician' part of my job was fitting and analysing 24 hour tapes.
These look like the old Walkman tape players but are connected to electrodes on your chest but do include a cassette tape - they are probably all digital now. Patients come to a clinic, we fit the tape, they go home or to work and have a normal day, the following day they come back, the tech puts the tape in a machine and we analyse the ECG at high speed and produce a report.
The cassette is then wiped using a magnet and is reused.
Background 2
We had a particular group of junior Drs who were a lot of fun and had acquired nicknames, one was nicknamed 'Biggles' because of how he spoke and started signing requests as Biggles. Biggles sent a request for a test on a patient who was 100+ and had a suspected assegai imbedded from the Zulu war.
Obviously fake so my manager sent a fake report back
So one day a couple of weeks later I removed a tape from a patient and start to analyse it. But I can't because it hadn't been wiped properly so I had 2 ECGs on it.
It happens, I had to phone the patient, make a new appointment, appologise etc. But when you superimpose 2 ECGs looks similar to a 'piggy back transplant' which is a heart transplant that leaves the original heart in situ.
Then (in my own time) wrote a report as though the patient had had a piggy back transplant, the patient's name I put Avril Fool, 1 Fool's Lane date of birth 1/4/50.
Because one of the recordings had a lot of abnormal rhythms it looked at first glance like something was seriously wrong.
Biggles completely bought it, was asking the medical secretaries to get the notes, the secretaries had twigged so were saying the notes were missing, was he sure she was a patient etc.
Fun times.
And no one was damaged, disadvantaged, had medical needs neglected.
Also when the patient came back I used a brand new tape.