Unsure about the changes : From my experiences whenever I comment the code it is because I am not sure about my changes for the time being and I want to test them first may be with some of the already existing pieces commented out. This is just to make sure I do not have to write the same stuff back if my changes do not work.
That is Ok to do as long as we realize that commenting out code is very temporary thing and should never cross the boundary of our workspace. If it does and it goes in the version control, then in a way we are doing what version control does for us (by letting us retrieve the older versions of the code).
Two or more scenarios out of which only one is applicable at a time: Make it configurable to pick one scenario at the run time rather then commenting out the valid scenario and uncommenting the invalid ones before each run. It might sound funny as to why people would want to do it this way but believe me I have seen such occurrences and to quote the most frequent is logging.
I will just leave it there commented out till my code is reviewed: Again just make sure it is not breaking the build and check it in. For the worst scenario, version control has the old version which we can always pickup anytime so why keep the dead code around.
I just read about it in an interesting post on code deletion. The Joy of Deletion
