The bug

Using the /op, /whitelist or /ban command causes the server to stop responding for some seconds while it is looking up a profile for a not loaded player.

The reason

The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.

The reason why this happens is because the method net.minecraft.server.management.PlayerProfileCache.getGameProfileForUsername(String) is called which tries to get the information from the Mojang servers.

Notes:

• This might be invalid as the commands return feedback on whether or not they ran successful
• This affects the following situations as well
  • Saving villages
  • Loading wolves, ocelots or horses
  • Creating player heads (see MC-65587)