Because this file is modified nearly every time a developer makes a major change to CVS, it's a primary source of conflicts -- here's an example conflict. Let's say I add the following lines to the top of the ChangeLog:

date 25 Feb 2001
 
This is the thing I added myself

However, let's say that before I'm able to commit these three new lines, another developer adds these lines to the top of the ChangeLog and commits his changes:

date 25 Feb 2001
 
This is the part added by another developer