I have configured the IDE to indent via tabs (size=2). Sometimes while editing files (especially in a shared development environment) I'll notice that someone used two spaces instead of a tab, but it's not readily apparent.
One solution is to show tabs visually, perhaps as a very light grey arrow. Or to be more clever, the IDE could only make tabs or spaces visible when they aren't expected, e.g. spaces indenting lines in a normally tab-indented file could show up as yellow, or similarly highlight tabs indenting a normally space-indented file.
Also, maybe visually draw tabs whenever they appear in the middle or end of a line (not as an indent). I don't think there is any reason for them to (ever?) be used there.
IMO duplicate of issue #4619.
*** This bug has been marked as a duplicate of bug 4619 ***