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Without reaching supermajority, multiple competing blockchains can cause a loss of confidence, insecure network due to miners splitting to other chains, confirmation issues, and will in general be bad. This is why any code that involves any sort of fork, contentious or not, requires a supermajority before the node begins to enforce the new rules.
Is there any code change in a hard fork that prevents miners from continuing to mine the old chain, like changing the magic bytes, or changing the version number and adding code that blocks nodes running an earlier version number?
I remember some cut and paste alt devs didn't change the magic bytes when they changed an old coin's code into their new coin's code. There were sync problems, and people said the new coin was trying to connect to the old coin's network because they both had the same magic bytes.