When generating chunks with the default Big Oak Trees (BigTree in the config), the big trees sometimes generate small. And not one of them, all do. I mean, like this: http://minecraft.gamepedia.com/File:Balloon_Oak_Tree.png
It often happens when the server is slow, for example after teleporting a few thousand blocks away. I've also encountered this issue when generating terrain with the fill option of World Border plugin.

I tried to fix this today. Our spawning code looks exactly the same as the vanilla spawn code, yet the results seem totally different.

I think this weird looking tree is considered as big tree in the minecraft code, because small trees look diffrent than that (in fact they all have the same leave-shape with some small variations in the corners).
No idea though why it could ONLY generate exactly this one.

Tonight I suddenly started getting this problem.

Before tonight, I never had this problem, now, suddenly, I always have this problem. Even when I return to the exact same world settings I had before, the problem does not go away.

Restarting my computer did not help, nor did reducing the background load on my CPU as much as possible - closing every other program, loading the world slowly, etc.

It was like someone flipped a switch. "Never create large oaks, ever = true." I have no idea what could explain this sudden problem for me.

ok, update on this

I've been testing a FromImage map using FillEmpty, but somehow the ImageMode setting got set to Mirror. I'm not sure how, I think it may have been set automatically.

Anyway, changing it from Mirror back to FillEmpty seems to have fixed the large oak thing.

The other thing I did was make my image offset exactly 50%, when it was slightly off before. I can't say for sure which of these two things fixed the oak problem, I would guess the ImageMode setting but I don't really know.