Exceptions from this rule are instructions manipulating the stack (push, pop, call, ret, enter and leave) which are implicitly 64-bit and their 32-bit counterparts are not available anymore, yet their 16-bit counterparts are. So:

  pushl %eax		# Illegal instruction
  pushq %rax		# 1 byte instruction encoded as pushl %eax in 32 bits
  pushq %r10		# 2 byte instruction encoded as pushl preceeded by REX.