This is more likely an oversight rather than a bug but during the Love Festival random flower bushes spawn. That works as normal but during my play time one of the bushes that spawned was from debug (cowplant spawner). It's an easy way to get a cowplant seed but I doubt that was intentional.
Additionally sims can sell painting that don't belong to them (or the household) on festival ground at least. That's more by design but it seems like a cheap way of making money (not that I mind).
Adding to this because I've seen it in my game, too. The paintings the OP is referring to are not the ones that appear on that board you can start street sales with. I've had my Sims tend those and start sales but if the paintings don't belong to them they receive no money. The paintings you can sell are the ones that random Sims paint and leave on regular easels that spawn at festivals or in the Art District. You Sim can sell any paintings left behind and receive money for them. Like the OP said, probably not a bug but just an oversight I suppose. My poor apartment Sim appreciates the extra money, especially since I have a few max level painters around town that come by and leave masterpieces behind lol... Though it does feel exploitative so I try not to do it unless my Sim really needs the money.
I wasn't able to duplicate it. My Sim wasn't able to sell those, just scratch.
Was your Sim a painter ?
My Sim is indeed a painter, but her brother who isn't can still sell them.
Willing to share your save game ?
If so, please zip up this folder: documents\electronicarts\thesims4\saves and drop the file to this Mediafire-Link to upload.
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