Hypothetically, an 8-speed manual transmission would need four shift gates, not to mention an extra cog for reverse, and the limited spacing between gears would increase the risk of bad shifting.
BMW's solution is to surround the gates with a magnetorheologic or electrorheologic fluid whose viscosity would change based on data from various sensors to physically block the engaging of certain gears. For instance, drivers would not be able to shift to 5th gear at 15 km/h or worse, shift to 1st gear at 120 km/h.
According to BMW, the technology can be used on a traditional manual transmission with up to eight gears as well as on a shift-by-wire (clutchless) transmission. Don't start waiting now, though, as patents can take years to see actual implementation.