Many times. For one building stuff is fun, running it often is not and while you are building it you get a much better picture of what it will take to run something once it is finished (aka: think before you leap).
Other reasons are that I realized something had already been done but much better even though my initial research didn't turn it up and finally because I could not get something to work at a level high enough to be exposed for public consumption.
Yes, several times. Sometimes opportunities feel like liabilities.