Prediction markets are fun, at least if you're making money. I've only been into them for a few months, but have already collected a bunch of interesting tales. Note: I may have been involved with some of these, but I'm telling these tales from a third person perspective.
One general point: all of these took place on Polymarket, a crypto prediction market. You can track which accounts place each bet, and so you can see their history of bets, but you can't tie it to an actual person unless they've chosen to identify themselves. You can look at the bet history at Polymarketwhales.info, although there's a ton of bets so it's easier if you know what you're looking for.
The Tesla market. Polymarket had a market on whether Tesla would announce a Bitcoin purchase by Mar 1, 2021. On January 27, an unknown user bet $60k on Yes. This was their only trade on the site, before or after. They won $180k, or 120k in profit. Odds are pretty good it was an insider. Is this insider trading? I asked Matt Levine but he didn't respond. Anyway, there’s another user that lost $242k betting that Tesla would not announce a Bitcoin purchase. This user is affectionately called the "Tesla whale” on the Polymarket discord. They're also notable for losing $92k on the super bowl the day before Tesla made the announcement, and they get honorable mention for having lost the most money on the 100 million vaccine market: see below. As of this writing, the Tesla whale is down nearly $500k.
Watch out for slippage: there was a market on whether Joe Biden would still be president as of Mar 1, 2021. Someone owned around 200k shares of Yes. The market price was very close to $1 each on the morning of Mar 1st, and they apparently decided to sell all their shares instead of waiting for it to resolve; however, there wasn't enough liquidity to sell them all at market price, and they ignored the warning about the slippage the order would encur. Their order ended up executing at an average price of 2 cents, and someone else scooped up those cheap shares a minute later, spending $1k to make $155k; talk about being in the right place at the right time! The initial user ended up with a total loss of $156k on this market. However, even taking that into account, as of this writing they're still up $175k, so don't feel too bad for them. (Note: it's possible they were trying to sell No shares to make a few cents and accidentally clicked on the sell Yes side; without confirmation from the user we can't know what they were intending. Regardless, if you've got hundreds of thousands of dollars at stake, double check before pressing buttons. This isn't the only fat finger that's happened, but it's the biggest.) Polymarket has since added a larger warning in red for trades that move the market more than a few cents.
Hashmasks. There was a market on whether the Hashmasks volume ranking on Opensea would be #1 as of Feb 28, 2021. Someone accumulated over 200k Yes shares, then took out a “flash loan” and purchased a Hashmasks from themselves for 130k ETH (worth over $100 million at the time). Unfortunately for them, Opensea doesn't count sales done directly through their smart contract, only one initiated through their website. The market resolved No.
CO2: Polymarket had a market on whether the level of CO2 reported by the Mauna Lua observatory would be over a specific value on a specific day. The number gets reported on the main page once a day, but someone found a page with the hourly data and figured out how to average together the data points to predict the daily number. They made around $10k on the first market and around $30k on the second market. After that, Polymarket stopped making new CO2 markets.
Vaccines: There was a market on whether there would be 100M 1st doses of covid vaccines reported by the CDC by 12PM EST on April 1st, 2021. The market was made before daylight savings time went into effect, and most people had assumed that Apr 1 data would not be posted by that time. Around two weeks before the deadline, the CDC started posting data earlier - in the 12:50:12:55 range ET. This was before 12 EST, so people realized that, if Apr 1 data was posted at that time, the market would technically include it. A week later, the CDC started releasing later in the day, and the market crashed hard. In the end, the CDC released April 1 numbers too late to count, and the numbers were just short of 100M anyway. We hit 100M on Apr 2nd, after 1PM. This market had $7 million in volume, and very nearly came down to what time the CDC happened to post the data on the last day.
Many people seemed to confuse the total doses given and what the market was actually about, which was first doses. On the day that total doses hit 100M, the Tesla whale spent over $200k on Yes at an average price of around 75 cents. There's been speculation that they thought it was over because total doses had hit 100M, but that can't be known for sure. However, multiple people showed up in the Discord that day asking for the market to be resolved since it hit 100M.
Souljaboy: There was a market on how many times Souljaboy would tweet during a given week. The way these markets are set up, they subtract the total number of tweets on the account at the beginning and end, so deletions can remove tweets. Someone went on his twitch stream, tipped a couple hundred dollars, and said he'd tip more if Soulja would delete a bunch of tweets. Soulja went on a deleting spree and the market went crazy. Multiple people made over 10k on this market; at least one person made 30k and at least one person lost 15k.