Shane Martin

Note:   it also works on cactus by merely zero-tick plusing a piston beside the block you want to grow, to easily reproduce a zero tick condition all you need to do is put a button on the side of a piston and click it.     The piston will start to extend, and pop the button off the piston which causes the piston to retract in the same tick.

http://prntscr.com/oqjdy0   You can just spam right click with buttons in hand and watch the cacti grow in the setup above

As shown in the video it’s highly exploitable and you might as well just give all the villagers the best trades by default, all librarians should just come with mending books for one emerald. It was the “intention” to make villager trading a bit more challenging? Now you don’t even have to get lucky to spawn the correct type of villager, any villager can be selected to become whatever job you want, then just break and remove their job that you pick until they start with the most desirable items.

right. so by that logic random mobs should be capable of choosing if they want to enter boats.   Sooo maybe a 50/50 chance they enter / get in it?   Because it would be impossible to say all non real life things would enter a boat 100% of the time.

Eh.. vex’s can fly through literally every block in the game. It makes zero sense for them to be stuck in a boat. If they can fly through oak planks it’s completely stupid for them to magically get stuck in a oak boat.

Okay, so this may not help the multitude of server owners that are having this issue but maybe they can use this to track down their problem and get it fixed..

1) This problem can be caused by a plugin trying to spawn a non-spawnable item stack. IE a plugin tries to spawn Material.wall_sign or Material.melon_stem (there are a BUNCH more) these are items that players aren't "allowed" to hold thus they become invalid and typically get rendered as smooth stone when they are actually dropped in the world.
2) Every time one of these non-spawnable items gets "dropped" in the world it will cause this error at least once. Often times it will constantly spam until the offending item is picked up and /clear is run on the player holding the item.
3) If you have a plugin that spawns random items from minecraft into your world that is causing this problem contact the plugin's author and have them add this little bit of code to their plugin to fix the issue.

You must have essentials installed and the plugin will need to hook essentials. (the plugin guys know how to do this)
ItemStack is = new ItemStack(Material_To_Spawn);

MetaItemStack metaStack = new MetaItemStack(is);
if(!metaStack.canSpawn(theirplugin.getES())) {
//System.out.println("Unspawnable item: " + is.getType().name());
//do whatever they need to do, eg: return/continue/break etc.
}