Tor does not verify a node's uptime and bandwidth advertisements, which allows remote attackers who operate a low resource node to make false claims of greater resources, which places the node into use for many circuits and compromises the anonymity of traffic sources and destinations.
http://www.cs.colorado.edu/department/publications/reports/docs/CU-CS-1025-07.pdf
http://archives.seul.org/or/talk/Feb-2007/msg00202.html