Ignoring to comment believing that you'll remember what you did.http://allthingsoracle.com/wp-content/uploads/2011/11/Oracle-development-comments-code.png
Well this is very common mistake, Many programmers follow the approach : "If code was hard to write, It must be harder to read".
This might be true in certain conditions but in most of the cases it becomes so hectic for the programmer (Coder) himself to recall some set of functions he used earlier. This is almost impossible to remember all the code you did while initiated the development even if you coded the whole project.
So it is a good practice to comment out the important lines of code for ease of access as well as to maintain the reusability.