The mixing usually happens by accident, you copy and paste bits and pieces, you need a lot of spaces for aligning things and just hit tab, you want to break the

The mixing usually happens by accident, you copy and paste bits and pieces, you need a lot of spaces for aligning things and just hit tab, you want to break the indentation rules by a space or two for readability, ... I am not saying that it can not work if you are careful enough, proabably with visible whitespace on all the time, in practice however it will become messy most of the time, especially as people working on a code base come and go.