tail -100 /somewhere/mylogfile > tmpfile
and the first line of tmpfile always has a bunch of garbage in it?
For this file with garbage in it (using the filenames in the example I gave above) execute these commands (you can use vim or vi)
You should now be at the last line of the file.
100 bsauve 7126 1 0 Nov 17 ? 7:49 uzposs2
You should see a 100 over in the left column - meaning your file has 100 lines. What do you see? Do you see 100?
Plus, what does the garbage first line look like?