Apparently there is confusion over which season is which, because the original Season 5 (Ninja Tribunal) was not aired after season 4, but instead was aired much later and then only in part. The reasons included trying to increase interest in the series and reduce violence. From Wikipedia's article on Season 5:
The "Ninja Tribunal" arc (13 episodes) was originally intended to be
season five of the 2003 Teenage Mutant Ninja Turtles animated series,
but the schedule was changed to try to increase interest in the series
and "Fast Forward" became the fifth season to air on commercial TV.
After the scheduling change was made, Mirage and its partners decided
to finish production on the "Ninja Tribunal" episodes and release them
directly to DVD. 4Kids Entertainment later signed a deal with Comcast
and this season began airing on Comcast-On-Demand in August 2006. Mass
confusion ensued regarding which season was officially season five and
which was season six. However after airing five episodes Comcast
stopped airing the episodes.
The "Ninja Tribunal" episodes were scheduled to be released on DVD
sometime in early 2007, but 4Kids Entertainment later removed them
from their release schedule. 4Kids started showing the episodes
starting on February 9, 2008. Although the title doesn't appear in the
show itself, the season was promoted in commercials as Teenage Mutant
Ninja Turtles: The Lost Episodes. The DVD set was released on May
20, 2008.
The "Nightmares Recycled" episode was never completed. The script was
finished and some animation had begun when 4Kids pulled the plug
because it was deemed too controversial and violent for a children's
program (Hun and the Garbageman were conjoined twins that were
surgically separated at birth by a back-alley surgeon with the
Garbageman being discarded as 'garbage'). Since there was no chance of
it airing, it was shelved in the early stages of production.
