If the output would be exceptionally long, $regsubex simply returns its unchanged input instead of giving an error. This leads to incorrect output in code that uses $regsubex.
Code:
alias binencode echo 6 -a * In binary: $regsubex($1-, /(.)/g, $base($asc(\t), 10, 2, 8))
I agree, this is a bug. If $regsub/$regsubex fail because the resulting line is too long, mirc should instead throw a "line too long" error as it does with all other identifiers.