Hi Kerstin, welcome to the Garden Party. I have not had many "worst" mistakes,thank goodness. But a whole lot of learning moments and missed opportunities.
We have a couple of apple trees, and early in my gardening experience, I used some of the cuttings as markers in my garden. A couple of the "sticks" took root and they budded and flowered. One was "planted" up-side-down and it still budded and flowered.
I did not want apple trees directly in my veggie garden, so I pulled out all the "sticks".
If that happened these days, I would have left the "sticks" in the ground as an experiment.

Oh I got one!

Last year among a ton of other stuff, I planted some heirloom âÂÂMexican Sour Gherkinâ cucumbers. They grew really well and started coming in in great numbers

I tried one and didn't really like the taste raw - our new kitchen was under construction so there was no way to try any recipes or pickle them so I gave them all to a friend to pickle.

I must have missed one and it grew for many more weeks. By the time I found under the leaves it seemed to have "magically" turn into a cantaloupe.

Well, perhaps it wasn't magic and I forgot I planted a few cantaloupe seeds and assumed the un-labeled plant with a creeping vines was the cuke.

Turns out I gave a whole bunch of un-ripe cantaloupes to a friend who then pickled them...

I still haven't had the nerve to tell her!

Welcome to the Garden Party, dzr4 .
I have realized one of my biggest mistakes is that I don't prune, trim, thin or shape my trees enough. I've found out that trimming a tree is a good thing. But it has taken years to learn the lesson.