SVN Blame/mercurial annotate
Very important feature when you are trying to figure out who is responsible for certain discrepancy within the code and bring their mistake to light, is svn blame (hg annotate, git blame).
Usage is quite simple – just type “svn blame source_file” and you’ll have per line changes of the current source file with the version of the line and who changed it last.
But to make it work properly, you need to agree within a project to have the same rules for trailing whitespaces, etc. So that during updating of source file members do not change other lines with their automatic tools.