The best I can recall is Daisy using a Cellular Shopper to buy a Cellular Shopper in Mario Party 3. Wasting 5 coins. I've seen it happen on more than one occasion. I've also seen the AI buy a Skeleton Key and then throw it away the next turn for seemingly no reason.
These. Their AI don't have anything about saving coins for items they don't want. Buy an item, or buy the same item for another? It's mind-boggling.
Another dumb moment is when a computer uses a Mushroom, Sluggish Shroom, or a Golden Mushroom to roll a number they want or a higher number, when the next space or two is a trap that stops them from continuing on. Or a trap where you could lose too many coins. Dumb.
Oh yes, I have seen them using mushrooms right behind a roadblock before. I don't think I have seen it with a Sluggish Shroom but from my experience they are fond of rolling 10s with them when a different number would have been more favorable than landing on a simple blue space.
The best I can recall is Daisy using a Cellular Shopper to buy a Cellular Shopper in Mario Party 3. Wasting 5 coins. I've seen it happen on more than one occasion. I've also seen the AI buy a Skeleton Key and then throw it away the next turn for seemingly no reason.
I've seen worse. The AI sometimes uses a cellular shopper, and exits out of the shop without buying anything.
Then it uses a Bowser Suit when no one is around them. They sometimes purposely land on Bowser when it's better for them to go the other way. I would understand it if they had 0 coins, but they never did. The AI also sometimes does chance time when they're in first and then screw themselves over by giving someone else the lead.
Those are just a few examples. I could write paragraphs on this thing, but I'm lazy. 
Ah yes, I have seen that plenty of times. However, I believe wasting coins on another Cellular Shopper while using one is worse than just wasting it. Although Daisy is probably the only character controlled by the AI that does that due to it being her favorite item while any character will do what you said so it's much more common.
They seem like they can't use some items properly at times like with that Bowser Suit example. Difficulty only affects their performance in mini-games so it can be bizarre seeing super hard computers doing this stuff. They almost never use Reverse Mushrooms on themselves despite it being so useful to go backwards at certain points.
Oh please do. It's interesting to read what stupidity the AI is capable of in the series.
I've seen even worse! In Mario Party DS, one time, the AI used the star pipe.. and the star was in the next space! 
Since then, I've learned how dumb an AI can be! 
I think I have seen them do that in Mario Party DS. I know in Mario Party 3 (I'm unsure about 2 and 4) they will do that if they manage to get enough coins to buy the star even if they are on the space right next to the star space. One smart thing the AI does do though is that they will use the Magic Lamp if someone buys or wins a Plunder Chest to prevent it from getting stolen so there's that.
