When Bitcoin-Qt detects that something is seriously wrong with the network, it disables most RPC functionality in order to prevent automated Bitcoin sites from losing a lot of money due to network problems. (The okSafeMode functions are not disabled.) All versions since ~0.3 go into safe mode when the longest chain is invalid (illegal transaction, creating too much money, etc.). Upcoming versions will also go into safe mode when a large fork (valid or invalid) is in danger of overtaking the current chain.