If you kill Scarecrow too quickly, he glitches, drops no loot, and you are unable to leave the room and instance. If you kill him before he transforms into a dog, he glitches. If you kill him before he disappears and reappears big, he glitches.
It will happen if You do overkilll dmg when he's at certain lvls of HP- players can do dmg to him for too long when he's about to switch phase and if god forbid it's two t4 dps pounding away at him he'll glitch every time- he's hp will drop to 0 before the final cutscene. There are other mobs that can be glitched that way tho it does not require repeating the fight for example the OMAC after the room with the 30 omacs in the DUO(You can drop him without him going to the shield phase), the trees in the Poison Ivy wing(They're supposed to explode at a certain lvl of health),Poison Ivy herself will ocasionally not go in her second Phase if she's hit with too much dmg, same goes for Bolt Claw in Loa.
Just happened to me. Mistyped the search, and thus posted a thread of my own, but yeah... This is exactly it. And since you're stuck in combat, the only way out is to either disconnect or to have someone outside of the boss fight kick you from the instance... Or to have the bats kill you very. Very. Slowly.
Had this issue last night. The part where Scarecrow disappears to lull you into a false sense of security, he never comes back. The terror level keeps rising. If you wait long enough, with the terror crows randomly descending on teammates, you can defeat the Nightmare, but still no Scarecrow, you're still in combat, and the door doesn't open. Glad we could disband. If you search the Arkham Asylum forum for Scarecrow, it seems this is a common occurrence.
A similar bug also happens in FOS1, when you do too much damage too fast. Then the boss get's stuck before the cutscene, where he walks a bit, can start, and becomes invincible. The only way to resolve this is to wipe and restart the boss fight. The most annoying part of those bugs is, that it somehow "punishes" people for being overgeared and killing the boss too quick, so this should be definatelly looked into and fixed.
I got a solution until they fix this. Those 125 Marks & T3 reward box and Metropolis Flat Screen that a swear drops 90% of the time in there aint running off no were.
