You don’t get an error because WordPress does not check for the length, and MySQL silently truncates it (giving a warning, not an error), unless you enable the STRICT_ALL_TABLES option (which will change the warning into an error).
Even more confusing, when you enable multisite the options are saved in the sitemeta table with a maximum key length of 255, but without multisite they go to options where the maximum key length is 64. Have fun debugging that in your plugin!