In the good old days even with T+4 settlements, fails to deliver...

In the good old days even with T+4 settlements, fails to deliver were a fact of life and regularily occured especially when short sellers were active. I haven't seen it in modern times when nobody uses certificates and T+2 is the standard. I recall back in the late 1990's when I happened to be very long a small illiquid stock listed in the US that was hit with a classic short squeeze after multiple failures to deliver. The brokers had been carrying the shorts (must have been good customers) and in the height of the squeeze the stock ran within hours from around $5 to over $20. I thought I had hit the jackpot and went into my on-line account (it was Ameritrade) and tried to sell my long position. As soon as I tried to place the order the stock ticker flashed up "invalid" and my order automatically cancelled. After a few attempts I gave up and tried calling the broker. I was given the run around and transferred around the office playing telephone tag for a couple of hours but never got any orders placed. The price dropped back hard and all my potential profit was gone. I complained officially to regulators who sided with the broker although they did conceed that trading had been "controlled" to "stabilise" the market. This was over 25 years ago.