Throughout the entire series, the computer seems to have only two levels of intelligence: "beyond human", or "IQ of negative eight". They will buy items for easy access to the Star, even if the cost of the item puts them below the coins needed for the Star. The computer will also use items to roll multiple dice blocks to get to the star when they don't have enough coins to buy a star. It gets worse on the investment boards like Windmillville in 7 and Koopa's Tycoon Town in 8. Most of the time, they will invest every single coin they have, even if it's not necessary or if they can't actually become the top investor, doing nothing but helping the other players. It makes it impossible for computers to invest on the building that is right next to it, unless they keep getting low rolls and winning minigames.
8 has King Boo's Haunted Hideaway, which is a randomly-generated map that changes each time you play it. The AI seems to not plan ahead at path forks, and it will choose a path even if it knows the next fork on that side has one path leading to a dead end and a Whomp blocking the other, and that it doesn't have enough coins to pay the Whomp's toll.
One particular case of the HARD AI in the first game being incompetent is pointed out by The Runaway Guys when Peach, otherwise a luck-manipulating bastard on Hard, proceeds to get the Ground Pound Coin Minigame and use a total of 9 ground pounds to find the 5 "correct" posts.note For those unable to watch the video, this is out of 12 posts, and all 12 of them were plainly revealed at the beginning of the minigame. The pattern went something like "pound sharp post, pound 'correct' post, repeat". It turns out that the AI is programmed to do this.
During their video of "Chilly Waters" the AI (Wario in this case) makes even more stupid decisions. He passes by the star multiple times and at one point, he duels Jon for one coin (by using a Dueling Glove he bought for 10 coins). Jon actually questions if the game is broken because of all the dumb decisions. And again, this was on hard mode!
In some games certain characters have "favorite" items they have a greater tendency to use. This leads to such hilarity as Peach buying a Plunder Chest and then wasting it because nobody else has any items; Luigi buying a Skeleton Key, never going near any of the locked gates, and then throwing the key away so he can buy another one; Yoshi using the Warp Block when he's the one closest to the Star; and Donkey Kong setting off a Bowser Bomb when he's right in Bowser's path and thus loses all of his coins. Daisy loves the Cellular Shopper, allowing her to access the shop from anywhere. So she buys a phone, uses the phone to buy another phone, uses that phone to buy another phone, and then uses her last phone and says "no thanks" because she doesn't have any more coins. Good job, Daisy.
In Getter Love!!, while you're pitted against three AI-players in normal mode, you'll have an advantage of your own in this already difficult game. For one thing, AI-players epically suck at mini-games, letting you win almost every. Single. Time! (Except for the whack-a-nerd game and the quiz game, the former which they're actually somewhat good at, the latter which requires you to understand what anyone says in the game.) Also, instead of using offensive cards strategically (like to fuck up someone else's date and possibly prevent them from declaring their love to someone for that turn), they tend to just use them on whoever has the most flourishing relationship with their girl of choice. Sometimes, if they challenge someone to a mini-game and win, one of their options includes changing their name, which has no effect on the gameplay, and they'll actually choose this every now and then. Most egregiously, whenever you talk to one of your opponents, you actually have the option to give them an item card or memory-unlocking topic, for no gain of your own, which they'll do every once in a while. Really, what's the point of doing that, or even such an option being there?
Any A.I. with an E rank will happily give you their property for what it's roughly worth should you offer. Even if that means increasing the value of your current properties to insane heights, making it possible to pay a lot more back in one turn. Of course, E-rank AI are Easier Than Easy as opponents go.
In the game released in the west (Boom Street or Fortune Street depending on your region) Slime has a while slew of idiotic behaviors, and intentionally so, being the board game equivalent of a punching bag for new players. Apart from taking balanced trades with no eye on the long term, Slime will only ever buy 20 stocks when he visits the bank, and only ever invest 200G into his shops at a time, leading to him swimming in ready cash by game's end. For the uninitiated, these games assess your progress by net worth, and there are very few situations where it's not advantageous to spend all your cash on stocks and sell them piecemeal as required - in fact Slime's failure to do this is why he often ends up lagging behind other training AIs.
Certain minigames in the first Wii adaptation of Schlag den Raab. A good example is "Wo liegt was?", where the players have to mark a specific location on a world map. The AI generally knows the location but, to give the human player a chance, is some pixels off – in any direction. So if you're supposed to mark the United Nations headquarters, the AI player might mark a spot that's clearly in the ocean.