Zappo
Explorer
As you guys in english-speaking countries say, hindsight is 20/20. 
So, what about sharing some of that? Errors you players have made which could have been avoided. Mistakes from which you should have learned.
Think about the last time one character of yours got killed, and tell us what you could have done to prevent it.
I've recently lost my half-elf bard to a poisoned crossbow bolt, and between the fact that I've mostly been a DM and me being a good player, I had kinda forgot how much it sucks to get killed. Especially with a bard, which IMO is the most survivable class of all, if played well (despite lacking damage-dealing capability, it has skills and powers to get out of almost anything).
The situation: after a fight with a tough undead, we had set up a camp to rest. During the combat, a few NPCs had fled, and we noticed that they were especially good at hiding and sneaking.
First mistake. We should have noticed that these guys were too stealthy for normal soldiers. And we should have wondered why they fled and didn't come back.
I told the elf in the party to guard us while we slept for most of the night, since she doesn't need to sleep much.
Second mistake. Never use only one guard. Also, just because you're an elf doesn't mean you've got good Spot and Listen.
We got attacked and she didn't notice until they were on us. They had poisoned weapons, with STR-hurting poison. One of them grabbed a girl NPC that was with us and started getting away. I cast expeditious retreat and went after him, planning to follow him as he fled and then using my bardic song and spells to stop him and interrogate him.
Third mistake. I didn't really need expeditious retreat. I should have used my wand of shield instead.
The guy, instead, dropped the girl and attacked me. He critted, too - ok, that was just bad luck. In my round, I attacked him with little success.
Fourth mistake. I severely underestimated these guys. I definitely should have gotten away after the crit.
Then, a hidden crossbowman shot me dead. The damage wasn't dangerous, but the 7 CON damage dropped my HP to a ridiculous amount, so that it killed me.
Fifth and final mistake. If these guys used Strength poison, they could use Constitution poison. I didn't think about this.

So, what about sharing some of that? Errors you players have made which could have been avoided. Mistakes from which you should have learned.
Think about the last time one character of yours got killed, and tell us what you could have done to prevent it.
I've recently lost my half-elf bard to a poisoned crossbow bolt, and between the fact that I've mostly been a DM and me being a good player, I had kinda forgot how much it sucks to get killed. Especially with a bard, which IMO is the most survivable class of all, if played well (despite lacking damage-dealing capability, it has skills and powers to get out of almost anything).
The situation: after a fight with a tough undead, we had set up a camp to rest. During the combat, a few NPCs had fled, and we noticed that they were especially good at hiding and sneaking.
First mistake. We should have noticed that these guys were too stealthy for normal soldiers. And we should have wondered why they fled and didn't come back.
I told the elf in the party to guard us while we slept for most of the night, since she doesn't need to sleep much.
Second mistake. Never use only one guard. Also, just because you're an elf doesn't mean you've got good Spot and Listen.
We got attacked and she didn't notice until they were on us. They had poisoned weapons, with STR-hurting poison. One of them grabbed a girl NPC that was with us and started getting away. I cast expeditious retreat and went after him, planning to follow him as he fled and then using my bardic song and spells to stop him and interrogate him.
Third mistake. I didn't really need expeditious retreat. I should have used my wand of shield instead.
The guy, instead, dropped the girl and attacked me. He critted, too - ok, that was just bad luck. In my round, I attacked him with little success.
Fourth mistake. I severely underestimated these guys. I definitely should have gotten away after the crit.
Then, a hidden crossbowman shot me dead. The damage wasn't dangerous, but the 7 CON damage dropped my HP to a ridiculous amount, so that it killed me.
Fifth and final mistake. If these guys used Strength poison, they could use Constitution poison. I didn't think about this.