Finding what has been changed in text, html, css, script etc file from the original backup to the new modified file really helps a lot reducing any error you might have made.
Recently I was facing some trouble with my .htaccess files and tried to look for where I did wrong. Later I found (by using the tool below) that there was one extra whitespace which screwed up my whole wordpress install.
DiffChecker (link) Find Text Differences Between Two Files
diffChecker is a free online diff tool that quickly and easily gives you the text differences between two files. Simply upload or paste the contents of two files and click ‘Find Difference!’ Uses : Compare text differences in two files. Find if document is tampered or not.
CompareMyFiles (link) Free Online File Compare Utility
Use to compare one file to another. Will show you a visual representation of the file and highlight the differences. The process is straightforward simply go to the site, select both files and click on Compare Files button. The application will compare files side by side and show you lines where they are different
Another similar utility to find differences in xml files etc is CompareMyDocs.