This idea refers to develop a program in the same way content in a Wiki is constructed: anybody can change it.

For the sake of simplicity, let's say the program is a spreadsheet, it is implemented client-side with no back-end. A platform similar to JSFiddle can provide access to the program and its source code, allowing every user to change it, with the changes made available to other users immediately.

Would this work? Is this a way to develop reliable, useful software? Interestingly enough, this is similar to how TikiWiki has been developed. Logic indicates it shouldn't work but hey, the same can be said about Wikipedia when it came out.

This is somewhat related also to some advanced MUDs with embedded programming languages, such as MOO.