This one is difficult to reproduce and difficult to pinpoint what exactly causes the error, but I have a few observations.

When using commands that analyze the tags of an Arrow entity e.g.

/scoreboard players tag @e[type=Arrow] add Meh {inGround:1b}

, if there are too many Arrows, Minecraft returns the following: "An unknown error occurred while attempting to perform this command". Such an observation can also be made with commands such as

/entitydata @e[type=Arrow] {...}

In addition to that, even if you only select a single Arrow entity, the error still occurs. e.g.

 /entitydata @e[type=Arrow,c=1] {...}

This only occurs if you have very many Arrow entities present (not extremely sure what the cut-off point is, but I reckon about 200).

Has also been tested with cows and zombies, does not happen. Not extremely sure about other entities. (Happens with Spectral Arrows)

STEPS TO REPRODUCE:

• Create a world (or just open one up)
• Shoot an arrow in the floor and run this command:

  /entitydata @e[type=Arrow] {}

  , and you should see: "The data tag did not change: {...}"

• Place down a repeating command block and input the following command: /summon Arrow ~ ~20 ~. Power the command block using any form of redstone for about 10 seconds and allow the arrows to fall.
• Run this command:

   /entitydata @e[type=Arrow] {}

  , and you will see: "An unknown error occurred while attempting to perform this command"

• This command will also trigger the error:

   /entitydata @e[type=Arrow,c=1] {}.

You also can't target them with /entitydata (/entitydata@e[type=Arrow] {} gives an error), but you can target them in /kill

See duplicate report (MC-93374)