Here's a list of some of my classic mistakes so you can avoid them:
A couple of times I've put supers on too soon and the weather got hot and the foundation crumpled up like someone had wadded it up. Then the bees used it. What a mess.
I once killed a queen marking her. I thought I was being quite gentle. Apparently I was wrong.
I once closed the top entrance on a hive that had a queen excluder and put in a triangular bee escape to clear the supers. Sounds simple enough, but when I got distraced and didn't take the supers off until several days later a layer of dead drones had clogged the queen excluder and all of the bees above the excluder were trapped in a couple of supers.
Once I put some supers on a triangular bee escape to clear the supers (usually works to get 90% of them out). Several days later, none of them had left. Several more days later and still none of them had left. Turns out the queen had layed some drone in the 7/11 comb I had and the bees would not leave while there was any brood in the boxes.
Once I heavily smoked the bees with tobacco in hopes of killing the mites. The bees asphixiated and were in a pile on the botom board.
A couple of times I've done a split and come back to check for queen cells. I thought I was being quite gentle, but all of the queen cells I found had been damaged by my manipulations. They are VERY fragile.
Once I was not paying attention to the kind of foundation that was sent to me for supers and was doing my usual method of not using a queen excluder and found out the hard way that the foundation was drone sized. The queen happily ran to the top super and layed it full of drone.
I used 9 frame spacers in the brood chamber once. Then I rearranged the brood a bit to open up some space in a honey bound brood chamber. The problem is that in 9 frame spacing the brood is the same depth but the honey sticks out a lot. I got some of that honey that sticks out a lot in front of brood the couldn't emerge and died.
I once killed an inordinate amount of bees with a wet/vac that was too powerful and it turned them into mush. I felt awful.
You probably don't mean them to be, but these suggestions sounded a bit humorous. I hope they are too you as well on hindsight.
Thanks for the tips!
It wasn't necessarily harmful to the bees, but was potentially harmful to me, and made me feel really stupid when I tried to move a hive that was secured with heavy straps instead of hive staples. The bottom board slipped, and the bees were really angry, and I got about 30 stings in every place my bee suit was tight against my skin. It postponed my hive moving for about a week, and really hurt, and was embarrassed that I had not thought about it more thoroughly.
Second dumbest thing I can remember doing was trying to place a new queen when I thought a new hive was queenless (the queen cage in the package was faulty, and the original queen flew away). I had been told the old queen would not likely return, but she apparently did, and I didn't check for her presence prior to placing the new queen in the hive. All the bees left except for 7-8 bees that stayed around with the new queen. Fortunately, I discovered this the day after putting the new queen in, and was able to take her back to the beekeeper I got her from.