Why is XmlSerializer adding junk to the end of my file?

I'm making a game and I am serializing the save game data. The data is simply a few lists of bools, ints and floats.

But about 50% of the time I get an error when the data tries to save or load that says "There is an error in XML document." The error is always located at the end of the file, even after it has changed size or I have added other variables.

It appears that very last part of the XML data is getting copied twice. The last line of the XML should read:

</LevelStats>

But when an error occurs it often reads

</LevelStats>>

Or

</LevelStats>tats>

Or some other small part of the last line duplicated.

Here is the class I am serializing:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Serialization;
using Microsoft.Xna.Framework.Storage;
using System.IO;

namespace Jetpack.Classes
{
    [Serializable]
    public struct LevelStats
    {
        // Player Bests
        public List<float?> fTimeList;
        public List<int?> iScoreList;
        public List<int?> iFuelList;

        // Time
        public List<bool> bBronzeTimeList;
        public List<bool> bSilverTimeList;
        public List<bool> bGoldTimeList;

        // Score
        public List<bool> bBronzeScoreList;
        public List<bool> bSilverScoreList;
        public List<bool> bGoldScoreList;

        // Fuel
        public List<bool> bBronzeFuelList;
        public List<bool> bSilverFuelList;
        public List<bool> bGoldFuelList;

        // Level Complete
        public List<bool> bIsLevelComplete;
    }
}

And here are my methods for saving the loading the data:

region Save & Load Level Stats
    public static void SaveLevelStats(LevelStats levelStats, string filename)
    {
        // Get the path of the save game
        string fullpath = System.IO.Path.Combine(Directory.GetCurrentDirectory(), filename);

        // Open the file, creating it if necessary
        FileStream stream = File.Open(fullpath, FileMode.OpenOrCreate);
        try
        {
            // Convert the object to XML data and put it in the stream
            XmlSerializer serializer = new XmlSerializer(typeof(LevelStats));
            serializer.Serialize(stream, levelStats);
        }
        finally
        {
            // Close the file
            stream.Close();
        }
    }

    public static LevelStats LoadLevelStats(string filename)
    {
        LevelStats levelStats;

        // Get the path of the save game
        string fullpath = System.IO.Path.Combine((Directory.GetCurrentDirectory()), filename);

        // Open the file
        FileStream stream = File.Open(fullpath, FileMode.OpenOrCreate,
        FileAccess.Read);
        try
        {
            // Read the data from the file
            XmlSerializer serializer = new XmlSerializer(typeof(LevelStats));
            levelStats = (LevelStats)serializer.Deserialize(stream);
        }
        finally
        {
            // Close the file
            stream.Close();
        }

        return (levelStats);
    }

    #endregion

Why am I getting the error above?