> Has this actually caused issues?
That was one of the issues for the clearinghouse with GME. When a market-maker sells short, they don't have to locate inventory prior to selling. Sometimes, they are unable to deliver to the clearinghouse, which technically forces the clearinghouse to remain short to the buyer. A "buy-in" is supposed to take place when a short seller cannot deliver, but this doesn't happen with market-makers because they get extra time under the rules, and also they tend to trade so much that the clock resets on their past undelivered shorts every time they buy and sell (this is called a "reset transaction" and it is illegal for any other market participant to do). If a market-maker gets caught wrong-way on a position where they sold lots of shares that they neither own nor have the solvency to buy at market, they can collapse and that puts the clearinghouse itself at risk. This is why a firm like Citadel (one of the biggest market-makers) had an incentive to capitalize Melvin Capital with $2B to attenuate a squeeze. IMO it was self-preservation, not charity nor opportunity.
The SEC makes FTD data available twice monthly. You can find it here: https://www.sec.gov/data/foiadocsfailsdatahtm
MBS wasn't a problem because of the defaults themselves. It was a problem because the correlation of defaults was underpriced, so banks took heavy losses on positions that they thought were pristine.
> Why? Buybacks are equivalent to paying dividends and then reinvesting them (which most people do).
This isn't true. Buybacks do two things that make them highly attractive to executives. First, buybacks allow option holders (including many insiders) to generate capital appreciation on those options without exercising them. Dividends, on the other hand, distribute only to shareholders and actually reduce the price of the stock (share price drops by the amount of dividend on ex-date). Second, buybacks provide a perpetual bid to the stock that mitigates the downward impact of selloffs, which is particularly nice for executives that receive performance incentive bonuses based on share price. With something like an ASR (a more complicated buyback traded against a derivatives desk), buying may even get more aggressive (in terms of % of total volume) as the stock declines. Buybacks are rationalized as capital return but really they are a form of inducing asset inflation.