There's something wrong with this...

First of all, I went to defend the Castle against the Gunners. All fine, except when they were all dead the settlers turned and started attacking me. Maybe I hit one accidentally, I thought. Reloaded, this time I went out the door to attack the Gunners, to minimise the chance of accidentally hitting a settler. We killed them all, the settlers started walking back to the Castle... then turned around and attacked me again! Fairly certain I didn't hit any of them during the Gunners fight. So I went far enough away that they stopped firing, made my way back and they were all like nothing had happened... WTF.

Secondly, and this has happened at least twice now, a "settlement is under attack" message pops up, fast travel there... no attackers. Settlers tell me they're glad I turned up when I did. Wait around a few minutes, still nothing, so fast travel somewhere else... and a message pops up about failing to defend. '___'

Then there's the time I killed all the attackers and still got told I failed it, once I'd fast travelled.

I don't get any of this, except that maybe I should (in the event of a successful defence) wait to fast travel until I've had the notification pop up.

YAaaaaay he dead wooo let party on is dead body LOL no I had that problem too I reload shot my settlement then win back reload again then it was fine it work like it should some times I got turn off my xbox if there bugs or back out to mean menus it work

The attacking system needs work; on console, i clear out an entire squad, no raiders left.. Yet it still claims wr're under attack. And theres inconsisitiencies with settlement supply-gathering, and glitches in the pipboy workshop screen.. Ect.

Itll get p
atched in time.

yea one time i went to defend a settlement and saw two settlers shoot each other - one of them was red to me so i shot him dead...the one that survived then proceeded to shoot the cow dead- but he still wasnt hostile to me....i talked to him and thats when the real attack from raiders started

wierd bug i guess - the whole attack system kinda svcks though to be honest

Next time you make a faction hostile mistakenly, just sheath your weapon

My problem is it pops up when I'm on the other side of the map and logically couldn't help. So I travel there and conveniently arrive just as they're attacking. Although if I dawdle, I'll fail the quest.

Until the settlement attacks are fixed, I have disabled settlement attacks (http://www.nexusmods.com/fallout4/mods/4180/?).

I do however use a scripted set of bat files to initiate settlement attacks on command. I''ve only done it twice so far, a small attack on Sanctuary and a medium attack on the Castle, just to see how the defenses held up. I was a little dismayed to see all my settlers run outside the castle to engage the attackers. Er... Why do I bother to set up elaborate defenses if my settlers charge outside of them?

As I said, I'm fairly certain I didn't hit anyone. The attack music stopped, the settlers started walking back, and then someone must have said, "It's her! In the metal suit! She's one of them!"
If I had accidentally hit someone they should still have been hostile when the last Gunner fell.

But point taken. If they do attack me again I'll sheathe my gun.

When "settlement is under attack" and no enemies it is OK. They comes later. Some times I wait 5 minutes, and I see enemies approaching.

This. There will still be enemies nearby advancing towards your settlement. It is a bit frustrating, but you'll just have to stick it out and wait for them to get to you (or hunt them down yourself). The enemies sometimes come in waves, you see.

Is it possible the settlers will turn if the happiness rating is low?

Settler grows tired and unhappy then decides the only way to handle this is with a mass shooting. Then you as the president have to get involved and start taking weapons away.

I fast travelled to Sanctuary and found the whole settlement including turrets fighting with one single guard(2 seconds battle). It has to be a bug. Its repeatable too.

Examine the dead body or,bodies. If they have a synth component, it is not a bug, badly as it was designed, and bugs out in other ways, it is itended design for some settlers to turn out to be synths

Yeaaaaaaaaaaaaaaaaaaaaaah, the AI in this regard has no means at all to demonstrate a behavior or an awareness of a settlement perimeter. Bethesda has managed to develop a wonderful little sandbox, but it comes with some pretty harsh limitations that I hope future mods can remedy (at which point I'd start to ponder: if a modder can do it, why not Bethesda? But I digress).

There's no mechanism for telling settlers to "not leave the camp and stay behind the defenses," and even if there was, there's no concept to the settlers for what those defense are or what "behind them" means. Everything you place is just an obstacle to be walked around, with no concept of what's "out" and what's "in."

That said, surely there's an easy work-a-round? The ability to place the bell or siren and assign a settler; in the case of attack there are two assignable settler behaviors; some will run TO the bell, others (guards) will keep the current behavior and will hunt down attackers. Tweaking the AI so that they closed the gates and stayed on the 'inside' of them or used wall/guardposts would be a bonus, for sure. Maybe a marker of sorts that designated guard rally points for the guard assign AI during attacks.

Thanks, good call. It was a synth indeed. Are they supposed to be spies or victims of "synthophobia"? Either way, i′m glad its not a bug.