At my current project we use SVN, but today I noticed a colleague of mine uses another version of version control as well.

Example:
V1:
console.log('Hello, world!');

V2:
//console.log('Hello, world!');
document.write('Hello, World!');

V3:
//console.log('Hello, world!');
window.alert('Hello, world!');
//document.write('Hello, World!');

Every singletime Ive to remove 40 lines of comments before Im able to follow the code and refactor it.