There are numerous correction mechanisms, and other mechanisms that destroy malformed proteins and mRNA chains. Wikipedia claims one out of every 1000 to 100000

There are numerous correction mechanisms, and other mechanisms that destroy malformed proteins and mRNA chains. Wikipedia claims one out of every 1000 to 100000 amino acids added to a forming protein is wrong. Each ribosome is proceeding to add amino acids to a chain at about 10 per second, and millions (or way more) proteins are being produced all the time.