That you can't cancel a transaction you have made. I have lost money bcs of that, and that was the day that I realize that btc has disadvantages. I know that I had to be very careful, but we are humans and we do mistakes bcs we are tired etc. But foe me that's the biggest disadvantages of bitcoin.
Like most people would say, your negligence isn't the fault of bitcoins unless the use of bitcoin somehow made you ignorant. In almost everything, you have to be sure before doing it. You should simply assume that in every payment methods, once your money was sent, then it's already sent. Paypal only added that feature because they don't really care if people are actually lying about getting hacked and not authorizing the transaction, hence, being able to dispute it.
The funds are all in Paypal's website and they don't mind bringing it back to your account. All they care about is their 2.9% + 0.30$ transaction fee, plus, many people are going to look for this "Sorry, I'm stupid. I mistakenly sent money to someone" feature. Business is business, they don't care about the scams caused by their dispute feature.