Detect tabs in Bukkit Yaml files.

YamlTabParser.java

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;

import org.bukkit.configuration.InvalidConfigurationException;
import org.bukkit.configuration.file.FileConfiguration;
import org.bukkit.configuration.file.YamlConfiguration;
import org.bukkit.plugin.java.JavaPlugin;

/**
 * Our class name is going to be YamlTabParser. Replace with your main class
 * name!
 * 
 */
public class YamlTabParser extends JavaPlugin {

	/*
	 * These two member variables are pointers to the config.yml file we will be
	 * creating. They are fields because they have a very broad scope and must
	 * be handled appropriately during save and load operations
	 */
	private File file;
	private FileConfiguration config;

	@Override
	public void onEnable() {
		// define member variables
		// NOTE: You cannot use a constructor in the main class! This would
		// crash your plugin!
		file = new File(getDataFolder(), "config.yml");
		config = new YamlConfiguration();

		// reload and save files, we will be overriding the JavaPlugin
		// implementations in a later step.
		reloadConfig();
		saveConfig();
	}

	/**
	 * Overrides by returning our defined FileConfiguration variable
	 */
	@Override
	public FileConfiguration getConfig() {
		return config;
	}

	/**
	 * Overrides the default save operation by saving using our predefined File
	 * and FileConfiguration variables.
	 */
	@Override
	public void saveConfig() {
		try {
			config.save(file);
		}
		catch (IOException e) {
			// print stacktrace if you prefer
			getLogger().severe(
					"Could not save config.yml due to: " + e.getMessage());
		}
	}

	/**
	 * Override default implementation for reloading
	 */
	@Override
	public void reloadConfig() {
		if (!file.exists()) {
			// Create file if it doesn't exist. Change appropriately based on
			// your setup
			saveDefaultConfig();
		}
		scanConfig();
	}

	/**
	 * Opens a java.util.Scanner to scan the config file for any tabs.
	 */
	private void scanConfig() {
		// declare our scanner variable
		Scanner scan = null;
		try {
			scan = new Scanner(file);

			int row = 0;
			String line = "";

			// iterate through the file line by line
			while (scan.hasNextLine()) {
				line = scan.nextLine();
				// add to the row
				row++;

				// If a tab is found ... \t = tab in regex
				if (line.indexOf("\t") != -1) {
					/*
					 * Tell the user where the tab is! We throw an
					 * IllegalArgumentException here.
					 */
					String error = ("Tab found in config-file on line # " + row + "!");
					throw new IllegalArgumentException(error);
				}
			}
			/*
			 * load the file, if tabs were found then this will never execute
			 * because of IllegalArgumentException
			 */
			config.load(file);
		}
		catch (FileNotFoundException e) {
			// this error should never happen if the file exists
			e.printStackTrace();
		}
		catch (IOException e) {
			// failed loading error
			e.printStackTrace();
		}
		catch (InvalidConfigurationException e) {
			// snakeyaml error if the config setup is incorrect.
			e.printStackTrace();
		}
		finally {
			// Close the scanner to avoid memory leaks.
			if (scan != null) {
				scan.close();
			}
		}
	}
}

Amazing! This is so useful! I'm gonna change this to automatically changes the tabs to four spaces, I recommend you add that as a comment to show people how to do that (just a simple replace and save to the config)