It's very common to find mixed tabs and spaces where people use tabs. The first sign of this is inconsistent indentation. Maybe they're just sloppy? Oh, hm, here's some more code where it's wildly inconsistent. Time to play guess-the-tab-width until it settles down. Now should I add a comment at the top to tell Emacs about this setting next time I open the file? Probably not, sigh.

Semantic tabs would be good with consistent culture and tooling behind them.