1. Not being prepared enough for diseases/ predators. BIO SECURITY
2. Coddling the cockerels hoping they'd grow up to be nice. This doesn't work. It just doesn't. Roosters should respect you and keep their distance. Spoiling them with attention leads to roosters that have no fear of God (you). I had a rooster who would attack and hump shoes and feet.
3. Not culling cockerels/ sick birds as needed. I "couldn't bring myself to do it". After culling three drakes and three roosters the flock is so much more peaceful and happy. Culling is always hard, but if you love them you must do it.
4. Chicken math. Its been said so many times on here but it is a real problem if not controlled! While waiting for my first flock of 11 (which i initially thought was a LOT) to lay, i added a whopping 20 more young pullets and cockerels, six ducks, and four guinea keets. THEN once my first flock began laying I incubated their eggs and got 18 more chicks! Thankfully my coop is huge and i have land so they all fit comfortably, but 50+ birds is too much. It becomes harder to give them one on one attention and daily health checks. You may miss the initial signs of sickness because you simply cannot keep up with everyone the same. Learn from me. Please. Just get a small flock and love them for a while
5. This could go with #3, but don't get too attached. Love them, appreciate them, care for them, but remember they are chickens. They're prey animals and very prone to disease. Sometimes you'll lose your favorite hen or you'll have to cull a dear rooster. Sometimes you'll lose one with little to no explanation why or how. These things happen when you keep chickens, and you must be mentally prepared for these situations.