It's nothing special that's not already pretty well known by now. It was seeing how popular loans that were some combination of zero documentation, baloon payments and jumbo loans.

People were getting multi-million dollar loans with no documentation which had affordable introductory payment and then ballooned to many times the payment.

I had no idea that wasn't normal but I was surprised when I learned that it was possible. I just lacked context and understanding of how the mortgage/banking industry really worked. Had I known as much as I do now after having been adulting for a while and become much more familiar with how banking really works, I could have seen that there was a massive pool of risk that was relying on the value of the housing market to not just keep increasing but increase at an incredible rate. All that it took was for the market to slow down just a little and then all those balloon payments would start defaulting because they couldn't be refinance again.

I couldn't have predicted everything that happened or who exactly would be holding the bag but, with just a couple more pieces of knowledge, I could have easily seen it was unsustainable.