Malicious relays may be able to be assigned Exit flag without exiting anywhere
The IANA for Multicast addresses indicates there are many /8's that are not yet allocated, such as 126.96.36.199-188.8.131.52.
The current voting mechanism in exit_policy_is_general_exit_helper allows an Exit flag to be assigned if it supports exiting to at least one /8 for 2 out of 3 ports of [80, 443, 6667]. exit_policy_is_general_exit_helper calls tor_addr_is_internal, this function only looks for the following IPv4 spaces: 10/8, 0/8, 127/8, 169.254/16, 172.16/12, 192.168/16.
A relay could put one of the unallocated IPv4 blocks and fool the Directory Authorities. Of course, if such a relay really wanted to do this, they could also set their relay up to exit to an uninteresting /8 no one would ever visit, such as one of the many military/DoD /8's.
Zack Weinberg's thread on tor-relays seems to have a good collection of addresses. Other sources are the exclude list from massscan and the IANA registry.
This would probably doubly true for IPv6, which only looks for fc00/7, fe80/10, fec0/10 - but right now exit_policy_is_general_exit_helper ignores IPv6.
 http://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml  https://lists.torproject.org/pipermail/tor-relays/2014-April/004431.html  https://github.com/robertdavidgraham/masscan/blob/master/data/exclude.conf  http://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml