Java Thread Race Condition

The main method in the MatrixModifier class starts a new thread to execute the run method and concurrently calls the printMatrix method. What is the primary synchronization issue that arises from this design?

public class MatrixModifier implements Runnable {
    private int[][] matrix = { {0,0,0}, {0,0,0}, {0,0,0} };

    public void run() {
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[i].length; j++) {
                matrix[i][j] = matrix[i][j] + 1;
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {}
            }
        }
    }

    public void printMatrix() {
        for (int[] row : matrix) {
            for (int value : row) {
                System.out.print(value + " ");
            }
        }
        System.out.println();
    }

    public static void main(String[] args) {
        MatrixModifier mod = new MatrixModifier();
        Thread modifier = new Thread(mod);
        modifier.start();
        mod.printMatrix();
    }
}