Here is the result of two checkouts: why the second is failing? 'git status' shows some files have been modified, but I am sure I haven't touched those files.
praveensripati@MyMini:~/Hadoop/Git/hadoop-common$ git checkout branch-0.21  
Switched to branch 'branch-0.21'  
praveensripati@MyMini:~/Hadoop/Git/hadoop-common$ git checkout branch-0.20  
error: The following untracked working tree files would be overwritten by checkout:  
    CHANGES.txt  
    LICENSE.txt  
    README.txt  
    bin/hadoop  
    bin/hadoop-daemon.sh  
    bin/hadoop-daemons.sh  
Please move or remove them before you can switch branches.  
Aborting
praveensripati@MyMini:~/Hadoop/Git/hadoop-common$ git status
# On branch trunk
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   CHANGES.txt
#   LICENSE.txt
#   README.txt
#   bin/
#   build.xml
#   conf/
#   lib/
#   site/
#   src/
nothing added to commit but untracked files present (use "git add" to track)
 
     
     
     
     
    