So I've been smurfing on this account at bronze for a while, and here's a list of common mistakes I see at bronze that you can either stop doing yourself to improve, or exploit the enemy doing it to carry.
1: Overextending late game without vision and dieing
Unless you have wards, it isn't worth it late game to go too far out and farm if you don't know where the enemy is, there's a large chance the enemy will group up and kill you, allowing them to easily go for an objective with no way of being stopped.
2: Facechecking Baron
A lot of times when your team suspects that the enemy is doing baron, people will pingspam and try to stop them. The vast majority of the time the stopping attempt is poorly coordinated, and the team attempting to stop them ends up dead. Try warding it from behind the pit, if you're on the side that can do that, or go in as a team with the tank leading to ward it if you do not know the enemy's location.
3: Defending Turrets
A lot of times what I'll see is people attempting to stop the enemy from taking a tower that they cannot defend. For some reason people have the mindset that they have to engage right before their turret falls, which never ends well. Sometimes, you have to just let the turret go. Another issue is people sticking around too long before the turret falls, so the enemy can just engage on you as the turret goes down allowing them to earn the turret and a kill, allowing them to take even more.
4. Sticking around too long after a push.
You killed the enemy team in a teamfight, and you're pushing. If baron is up, you need to back away and get home after the push. Don't run around stealing their jungle, the enemy is at full health and bought up now, and believe it or not they can rush baron if you stick around too much, since you are likely weakened from the last fight/push and are likely backing too late.
5. Letting people know they're in a warded bush.
Saying "Hi,[name of champion]" isn't the only thing you can do wrong, either. if you fire a blind skillshot into a bush, chances are the enemy will know you see them and will back off. This may seem like a good thing, but often it's not. If the enemy jungler is camping your lane, they are losing exp and farm while they remain there.
Additionally, this can be very bad later on too. If the enemy is in a warded bush and are unaware, you may be able to take advantage of it. Here's an example:
I'm playing support leona, and I see the enemy team all clumped together in a warded bush, next to wraiths. Right as I'm typing to let my team know I'm going to ult the bush to initiate, and to come here fast, our ezreal decides to get in some free poke with a mystic shot, so all 5 of them back away, ruining our perfect opportunity for an easy teamfight.
6: Giving away jungler ganks
When your jungler is ganking, he may want toinitiate, or maybe you will. It really depends on what champion everyone is playing. Regardless, don't make it obvious. If you run straight at the enemy mindlessly, if he's any good he will realize that the jungler is ganking, no one mindlessly walks like that alone. Wait for your enemy to get into a position where you can initiate(damage trade last hit etc), and then do it.
7: Diving the enemy ad carry/not protecting yours.
At bronze, everyone has the mentality rhat everyone should mindlessly dive the enemy adc. It sounds reasonable, since their adc does the most damage and is the squishiest, right? The thing is, the enemy team has the exact same mindset, to dive yours. As the ad carry, stay a safe distance in all scenarios. The only time you shouldn't be focusing the closest target is if you can safely focus a further one that's more important with no risk. if you try to run in and focus their ad carry, not only can their ad carry focus you, but their entire team can as well and you will get nuked down.
As anyone else, you need to learn to prioritize. Try and help prevent the enemies from reaching your ad carry, abilities like nunu's iceball, morgana's snare, and maokai's root help a ton at keeping people off of your carry. If you don't have much peeling ability, and think you can take out their carry, you can go ahead, especially if their carry is in a bad position.
I honestly can't explain this perfectly, as it's different in so many situations, it just takes experience. Point is, it's not always the best idea for everyone to dive in on the enemy ad carry every time.
8: Actions done with your lane is missing.
So your laner is missing, and you don't know where they're headed. Even though people should be watching the map, and people that require mia's to be aware are bad, you should call it anyways. Many people at bronze do not watch the map, so calling mia for your lane/other lanes can help them out, even though they shouldn't need the help in the first place.
Additionally, here's a mistakes I personally used to make a lot, and others as well. If you're mid, and you know the enemy mid laner is ganking a certain lane, don't follow them directly. Go through your jungle, otherwise you may meet their jungler coming across and die yourself, additionally you're are less likely to be spotted by wards. because of this, you can very easily bait them into a tower dive, and pull off an easy triple kill if executed correctly(you wont believe how many triple kills I get from this)
9: wasting ults/summoner spells when dieing.
If you're going to die sometimes you just need to accept that their is no escape, assuming their is none. Say you're at 50 hp and just got ignited, theirs no reason to flash away, you're gonna die anyways. Now you're still dead, and the enemy jungler can camp you now since you don't have flash, so you'll likely die again.
10: Playing too agressively when behind.
I see this a ton. People are upset when they die, and they feel like they have to make up for the death by getting a kill. This is why games are never close at bronze, and always snowball. If you die, accept that you're behind and just farm up until they make a stupid mistake or the jungler ganks. People throw so much at bronze, mistakes are rampant so in all honesty a game is never lost until the nexus explodes, if you fall behind you will be able to make it up at some point, just be patient. Even if your team is feeding too, there's a fair chance the enemy will do something stupid late game and you will make a comeback, just be patient.
I hope these help