5 bytes are required in insert "CALL Hook_Bridge instruction" (1 byte for command and 4 bytes for offset). So at least 5 bytes are always overwritten. Erm Hooker plugin relocates overwritten commands to new memory buffer, preserving all of them. But if there are jumps from other code to overwritten bytes, you'll get a crash. Because this external code will jump into the middle of new CALL command and will interpret new bytes as normal commands. So generally it's safe to overwrite group of assembler instructions with at least 5 bytes in length (like PUSH something, ADD something, XOR something, CMP something), not in the end of function and without jumps in the middle of those 5 bytes.