Many version control systems use some kind of keyword substitution, e.g. in Subversion the keyword $Author:$ gets replaced on every commit through the committing author, say $Author: crodemeyer$. Now imagine that you have several branches of your trunk and that each branch is modified in parallel. When the time comes to merge back the changes to the trunk you will get many conflicts. They cannot be resolved automatically because in every branch the same line of code has been changed. Indeed you have entirely broken automatic reintegration of branches if you use keyword replacement.
Fortunately Subversion has the possibility to control keyword substitution with the svn:keywords property. Be a hero and use “svn pd svn:keywords –R” to disable keyword substitution completely.