I think most of mine were around the people skills thing.

Not grasping the true meaning of what "its not what you know, its who you know" thing and "People and processes over tools and technology".

Being right is no substitute for being believed. Going the extra mile is all very well but if no-one cares then you've burnt some family time doing the impossible for the ungrateful.

Sometimes people are hell bent on doing something stupid. Sometimes less damage is done by letting them get on with it, having their inevitable disaster and either have them learn from it or move on. Trying to prevent the disaster actually prolongs the impact and you can end up being tarred with the wrong brush and blamed. I've made this mistake a few times and ended up being ill because of it. Make sure you are ready for the disaster and have plan B warmed up and ready to go.

I joined IT at a point where people in IT cared that the job was done right. It was a vocation for pretty much everyone involved. It has come as a rude awakening to find that some people just regard it as a job an scoot along doing the bare minimum, collecting their pay cheque at the end of the month.

My uncle said "don't work for losers, try and work for people who are going somewhere". His rationale was that a successful person will want to take his team with him. A loser will get replaced and it is more likely that they will be replaced by an external person (who wants to bring his own team in) than promote from within. You don't promote in a losing team.

Spending too long in a niche subject area. Have a principal skill topped up with niche supplement. That way if the niche closes you have at least some relevant skills on your CV.

Don't go chasing every technology fad. There isn't time to learn it all and you'll burn out trying. Focus on learning the concepts behind the tech and what the tech was designed to be used for. Some things come back into fashion and some concepts give you down different perspectives on how to use the tech you already know.

Above all remember that you spend a huge amount of time at work. If you don't enjoy it then you are in the wrong job. Many of our prisons are in our mind and of our own making.