In the fall of 2012, I had a friend who owed me $21,900. Martin Shkreli owed him a similar amount, so he told Shkreli to just send me a check for $21,900. Shkreli then wrote out a check to me, photocopied it, and sent the picture to my friend as "proof" that he was paying me. My friend then forwarded me the email Shkreli had sent him.
Although I had never heard of Shkreli before this, I knew the minute I got that email that Shkreli would not send the check. If he had intended to pay, he would have simply sent the check. No one would go to the bother of photocopying it and sending the picture if he actually intended to pay.
I couldn't really say anything to my friend at that point because he was operating in good faith, and I didn't know anything about Shkreli, but I knew I wouldn't be getting it.
Three weeks later I told my friend that Shkreli hadn't sent the check, and my friend apologized and sent me a check himself.
I have no idea whether Shkreli ever paid my friend, as my friendship with him ended soon after.
But I haven't been all that surprised by what I've read about Shkreli since. Like all sociopaths, he thinks he's fooling people when he's not, and I have to admit I was gratified to hear that the Feds caught up with him yesterday.