In 2.6, Ref Manual / Lexical Analysis / Indentation says

"First, tabs are replaced (from left to right) by one to eight spaces
such that the total number of characters up to and including the
replacement is a multiple of eight (this is intended to be the same rule
as used by Unix). The total number of spaces preceding the first
non-blank character then determines the line’s indentation. "

and Using Python / Command line / Misc options says

"-t
    Issue a warning when a source file mixes tabs and spaces for
indentation in a way that makes it depend on the worth of a tab
expressed in spaces. Issue an error when the option is given twice (-tt)."

Inyeol Lee, on c.l.p., in response to a report of a TabError, says "-tt
option in python 2.x is now default in python 3.0.
Apparently it got slipped from any documentation, including what's
new."

In 3.0 docs, the -t option *was* removed from Using Python, but the
Indentation sentences quoted above remain unchanged, though obsolete.  I
would guess that they should be replaced by something like

"When a source file mixes tabs and spaces for indentation in a way that
makes it depend on the worth of a tab expressed in spaces, a TabError is
raised." 

Perhaps What's New should get a sentence too.  "The former -tt startup
option to raise TabError on ambiguous mixtures of tabs and spaces for
indentation has been removed as an option and is now the standard behavior."