Every club will have their stories of regrets - of players they let go.
It just seems to Arsenal fans that they have more than most.
The likes of Ismael Bennacer and Jeff Reine-Adelaide have just earned big moves to top European sides, but Serge Gnabry would appear to be the one big mistake that the Gunners have made in the last few years.
But there could be another name to add to that list now too - Donyell Malen.
When Arsenal plucked him from the Ajax academy in the summer of 2015 there was a considerable amount of hype and expectation surrounding the prospect.
A player that Ajax really didn't want to lose, the striker made an impressive instant impact at Arsenal as he produced some encouraging performances at U18 level and quickly made the step up to what was then the U21s.
However, Malen didn't push on as expected at the Emirates and was sold to PSV Eindhoven in 2017.
Since then, he has gone from strength to strength and broke into the Netherlands squad this month, scoring on his debut in the 4-2 Euro 2020 qualifying win over Germany at the start of September.
And now he has left Arsenal fans wondering just why they let him go, with a stunning performance in the Eredivisie for PSV.
PSV thumped Vitesse Arnhem 5-0 on Saturday, with Malen scoring all five goals.
After one goal in the previous four game of the season, it was quite the return from the former Arsenal academy product.
And Gunners supporters on Twitter were asking whether or not they had a buy-back option in the contract that took him to PSV.
Not only that, but with Gnabry starring for Bayern and Germany and Joachim Low calling him a "pivotal player", the Gunners faithful are left scratching their heads.
