Or if you have your tabs set at 3 and I have mine set to 4 and you want to indent something 6 characters so you enter two tabs. When the code gets back to me it's indented 8 spaces and nothing lines up anymore. So I change it to one tab and 2 spaces and when it gets back to you it's indented 5 spaces instead of 6, so you change it back to two tabs and we spend our time indenting code to make it readable instead of doing our jobs

We had such vicious battles at Convex we wrote a tool that "canonicalized" code before it was checked in and then "decanonicalized" after checking out. There were issues with comments, but they were manageable.