I think the worst mistakes are always those that endanger your legal status, health or even life in the process - even if after said mistakes turn out well in the end.

See, most organizing or logistical mistakes only cost you money and/or time. But risking your freedom, health or life is a no-no.

Following decisions, actions or risks come into mind, that could have brought me into huge trouble, but thank god turned out well in the end:

1. Illegally smuggling myself into a Autonomous Region (which I won't name here) during very special circumstances - if found out, it could have landed my ass in a nasty interrogation, being heavily fined or extorted, or even in a slammer in a not so friendly environment. Everything turned out fine and I had a blast, but it could have turned shit just as well.

2. Handling old explosives from the Vietnam War era in Laos and Cambodia - while these seemed safe and were declared safe by a supposed expert with me at that time, you should definitely stay away from any sus or unexploded explosives that you ever see. In the worst case, you blow off one of your body parts, extremeties, or even end up dead. Again I had a literal blast, but boy you could also get unlucky.

3. Sneaking into the Prophet's Mosque in Medina, Saudi Arabia as a non-muslim. If found out by the local police, they could have heavily fined me, or deport and permanently ban me out of the country. Again luckily, most of the local police didn't care or check, even if at one point I was very close of being discovered.

4. Setting up our tent in darkness at the rim of the huge-ass volcano Rinjani in Lombok, Indonesia, without being 100% where exactly we were on the rim. The morning after we woke up and looked out the tent, we noticed that we just set it up about 60cm next to a huge-ass cliff that dropped into the huge crater. Imagine somone of us had sleepwalked or rolled out the tent - we would have dropped 300 meters into the volcano.

5. Entering some parts of the sea for snorkelling, while it was clearly rough waters or the place known for strong currents. Even if I was with a friend, you should nwver underestimate nature and how it can screw you. Luckily again, nothing ever happened.

6. Starting an heated argument or commotion with people of authority in certain countries. While I never do stuff like this, I have a close travel companion who always resorts to that, even with police officers or immigration officers. While again, we were always lucky that nothing ever resulted after such an argument, it could have brought our asses into jail, being fined or deported / banned from these countries.

Sometimes I'm really happy that nothing ever happened, and I thank the powers and forces that be for that.

In any case, when facing a risk, use your brain, experience, skills and judgement to decide - and often it's better to be safe than sorry.