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AI sometimes holds on to certain cards that it will play in other situations, but I can't find a reason. For example, AI has Stone Rain as its only card and plenty of mana, but holds on to it even though it now casts Stone Rain a lot otherwise. Also, AI is holding 3x Bargain, has no blockers for my on-board kill, but does not cast Bargain even though at other times it will.

I observed a very poor mulligan decision (or rather lack thereof): AI is playing a deck with 4x Mountain and only 4 red cards. Keeps hand with 3x Mountain, no other lands and no red cards. Completely unplayable.

The AI is too apprehensive about Alaborn Zealot. It's good that it doesn't suicide its bigger threats into it, but it will actually freeze up and hold off on attacking entirely even when it's just my little Alaborn Zealot holding back a giant army.

AI stops tapping its Talas Researcher, presumably because it already has enough cards in hand. It is, however, stuck on lands, so it should still draw. In fact, it should always draw. Looting is better than not looting.

AI casts Glorybringer (uh-oh) but then immediately taps it to transform its Town Gossipmonger and passes the turn.

AI casts Nissa, Voice of Zendikar (post-combat) but passes the turn without activating it.

AI has 6 mana and I have a 3/3 and 4/4 creature with horsemanship. AI casts Borrowing the East Wind X=3 instead of X=4.

If I have a Grafdigger's Cage in play, and the AI has something with unearth in its graveyard, for example Sedraxis Specter, the AI will spend mana to unearth it multiple times per turn. Even though it is prevented from entering the battlefield from the graveyard each time by the cage the AI will effectively waste all its mana on this for the rest of the game.

The AI attacked with a Time Elemental even though it had a power of 0.

AI has problems playing against Iroas, God of Victory. It insists on double-blocking my indestructible attacker with creatures that just die to that, even though it is at a healthy life total.

I observed multiple times that the AI is extremely reluctant to activate certain creature abilities... except when it's attacking for lethal. Setessan Griffin, for instance, won't ever be activated no matter how much mana there is to spare unless it's attacking for lethal.

Some AI misplays I observed when playing against the quest world opponent Neith
-Used Mystical Tutor and Enlightened Tutor in the same turn wasting the first one
-Didn't sacrifice any lands in reaction to Armageddon being on the stack
-Didn't sac Mirror Universe at 7 life vs my 20, didn't live for another turn due think about it again. (in a later game, it did it at 11 vs 20 though)
-Don't think I've ever seen the AI use the mana gained from Mana Drain to cast anything.

And something else :
When I tapped Preacher, the AI had two identical creatures, one he enchanted by Curiosity, the other he did not. He gave me the enchanted one, maybe because it was tapped - it should give away the unechanted creature in this case.

My AI opponent just played Xenagos the Reveler (second main phase) and passed the turn without even activating it.

If an opponent controls six Swampss and two Forestss, and would sacrifice one of them after, say, they play Harvester Troll, it would be better for them if they sacrifice the land that they have more copies of.

Some cards, for exemple Rainbow Efreet, can evade threats like Vindicate just using their innate abilities, still the AI will try all time to use counterspells before for no reason.
Another time that the AI use counterspells for no reason is when the target there is a creature alike to Phantasmal Bear since the creature dies anyway. Thank a lot, you people here do a great work.