Q:

newby issue: where does my commit disappear

I'm very very new to git and just started playing with it. 
That's what I did:
john@john:~/workspace/git-test/tmp$ git log --pretty=oneline

7d6bebed1d7bcfd916dd930303b7e95c974ad354 xThis will be amended
2c32b4519c8c06e6e6ae381a67d6824a33a512ce another emty file
cd34e9e0c40f18984f66e58bd1650c98f8c3aedf Merge branch 'master' of /home/john/workspace/git-test/../tmp
408020f5e0ffda578e8690a487c80f3c22ea2804 Empty file added
4b80be932603083d9712aa4329812d045d22eafa added emty file
7ab109a5fe405dcb9f52424a8b27790b51ff43bf Testing .gitignore?
92ba23c892102e8162b700a43548567cefe7183a Initial setup

Than I tried to switch to a previous commit:
john@john:~/workspace/git-test/tmp$ git checkout -b issue01 408020f

Switched to a new branch 'issue01'

And check the logs:
john@john:~/workspace/git-test/tmp$ git log --pretty=oneline

408020f5e0ffda578e8690a487c80f3c22ea2804 Empty file added
7ab109a5fe405dcb9f52424a8b27790b51ff43bf Testing .gitignore?
92ba23c892102e8162b700a43548567cefe7183a Initial setup

Question:
Where is this commit  4b80be932603083d9712aa4329812d045d22eafa added emty file?
I'm pretty sure I did something wrong, just don't know where to check for it...
UPDATE
john@john:~/workspace/git-test/tmp$ git log --pretty=oneline --graph
* 408020f5e0ffda578e8690a487c80f3c22ea2804 Empty file added
* 7ab109a5fe405dcb9f52424a8b27790b51ff43bf Testing .gitignore?
* 92ba23c892102e8162b700a43548567cefe7183a Initial setup

john@john:~/workspace/git-test/tmp$ git checkout master
Switched to branch 'master'
Your branch is ahead of 'origin/master' by 4 commits.
john@john:~/workspace/git-test/tmp$ git log --pretty=oneline --graph
* 7d6bebed1d7bcfd916dd930303b7e95c974ad354 xThis will be amended
* 2c32b4519c8c06e6e6ae381a67d6824a33a512ce another emty file
*   cd34e9e0c40f18984f66e58bd1650c98f8c3aedf Merge branch 'master' of /home/john/workspace/git-t
|\  
| * 4b80be932603083d9712aa4329812d045d22eafa added emty file
* | 408020f5e0ffda578e8690a487c80f3c22ea2804 Empty file added
|/  
* 7ab109a5fe405dcb9f52424a8b27790b51ff43bf Testing .gitignore?
* 92ba23c892102e8162b700a43548567cefe7183a Initial setup

A:

git log <commit> shows the history reachable from <commit>. If you look at your history, you'll see that 4b80be9 added emty file isn't an ancestor of 408020f, thus it won't show up in the output of git log 408020f. You only see it in the first command's output the way you do because --oneline puts the commits in chronological order, even though the history isn't linear.
Checkout the git-scm docs about branching and merging. They're great and explain these concepts well.

