In vanilla Minecraft with no mods, one can issue this command to get coordinates of a player:
/tp player ~ ~ ~
When the Essentials plugin is loaded by the server, there is no code to deal with this request. It instead runs this code in the commandtp.class file (Line 90):
User target2 = this.getPlayer(server, user, args, 0);
double x = args.startsWith("~")?target2.getLocation().getX() + (double)Integer.parseInt(args.substring(1))double)Integer.parseInt(args);
This throws an exception (handled) because the code is attempting to make "~" characters into integers.
The player is given this message on screen:
Error: For input string: ""