You can not 'mess up' your ECG.
It is nowt more than a recording of the electrical activity of your heart, bit like taking a picture of you.
There are a gazillion reasons why the ECG can not look like a bog standard one. The art is in differentiating between what is a variant and what is an expression of an abnormality or disease.
That sometimes requires additional tests.
After passing the intial medical few people drop out but can do so for the usual problems people develop later in life.
Interestingly enough I read somewhere that the most common cause for loss of medicals in the States is chain saw accidents. Not been able to verify that yet but seemed an interesting snippet.