Dagor Aglareb
The Third battle seems to feature another example of poor intel
reconaissance by Morgoth while the Noldor (aided by Ulmo) appear
to be thinking several steps ahead of M.- Turgon and Finrod being
inspired to begin their search for refuges while M.'s spies misread
the situation in Beleriand, believing that the Noldor are "wandering
abroad with little thought to war". Morgoth tries to use shock and
awe (volcanoes and earthquakes) and then sends raiding parties east
and west with the main effort in Dorthonion. But good Noldor intel and
preparation (prior planning prevents poor performance
) leads to
Noldor special forces eliminating the orc raiders while the nassty
orcsees in Dorthonion are destroyed.
However, a glaring planning error of the elves seems to be their
neglecting to plan for an effective siege and seizing of Angband
(and how else will they get back the Silmarils?
As for Morgoth, in the Dagor Aglareb he seems remarkably
unimaginative. Both elves and Morgoth here seem in somewhat of
a World War I mentality, not thinking creatively For example,
ignoring tactical implimentation failures, Churchill with
tanks and the Dardanelles ploy).
However, Morgoth does seem to learn from his errors. He significantly
upgrades his intel, both through more effective use of spies and a form
of "brainwashing" of captured elves, and his developing new weapons
systems.
And by the by, why were Noldorin elves deterred by ice and snow north of
Angbad, when you recall Leggy's walking on snow at Caradhras, and
being uneffected by the cold?