I heard flash bitcoins (Fake bitcoins) are circulating.
They disappear after 90 days.
Now, if these do exist, crypto market will crash!
Anyone see this as a threat?
Everyone denying?
You should read this article, and I am quite sure that you will find your answer in this article.
I was not aware of the flash bitcoin, but if they are some fake shit tokens, like I have many of them in my wallet, which is used less and are not tradeable and most of them can lead us to huge scams, like if we will connect our wallet to sell those tokens we might be breached by hackers or scammers. In other words, selling these shit tokens or coins can lead to big losses.
So, the best you can do is, compare the BTC smart contract with the token you are receiving, and check if that's real or fake. You might be scammed even if you will check those token transactions on block explorers because transactions of shit tokens are also recorded on the explorers. So do not rely on them only and verify the nature of the BTC manually.
You need to get some information on how cryptography works and to know that, you should read the above article.