Here's a quick short list. These are mistakes I made only once, or eventually stopped making. Some are mistakes that came about as overcompensation for previous mistakes:
1) Believing HR's lies.
2) Transmitting HR's lies to my employees, unfiltered.
3) Getting HR involved in stuff that they only make worse (a function of #1).
4) Not believing anything HR says at all.
5) Not transmitting HR's policies and/or requests to my employees.
6) Not getting HR involved, when they should have been involved.
7) Making off-color jokes. OK when a grunt; NOT OK when a manager.
8) Getting too nosy about employees' personal lives.
9) Not getting nosy enough about employees personal lives.
10) Projecting my personal values and motivations onto employees.
11) Projecting my personal values and motivations onto other managers, or my managers.
12) Trusting too much.
13) Not trusting enough.
14) Refusing to delegate.
15) Overdelegating. Putting too much responsibility on the shoulders of people that shouldn't have to deal with it.
16) Putting the burden of progress reporting into basic team policy. It needed to be my job to know progress; not my team's to tell me. This is what scanning commit comments and build results is good for.
17) Not respecting different cultures within the team. I did fine with cultures outside the team, but each of my team members had their own personal culture, and I needed to get familiar with (and respectful of) each team members' personal culture.
18) Putting my team's needs before corporate needs. The #1 job of a manager is to make sure the corporation's needs are met; regardless of whatever else is going on.
19) Putting my needs ahead of my team's needs. I learned that I needed to make sure the team, and its costituents, were healthy, and had its needs met, BEFORE my own.
20) Being afraid to say no.
21) Being afraid to say yes.
22) Avoiding taking personal responsibility for the team's performance.
23) Throwing my employees under the bus, in order to save my own butt.
24) Letting employees get away with crap, until it became an emergency.
25) Thinking that my job was all technical. In reality, the tech was only a tiny part of it.
As you can see, "hard and fast rules" are not particularly helpful. It really is a human exercise.