If a filter is removed from a service at runtime and there are open sessions, the behavior of the session can change in ways that the existing filter sessions do not expect. In practice this is only a problem for services that use readconnroute with filters that require more complex processing e.g. the maxrows or cache filters. This can also affect other routers but it is less likely to cause problems as they already require a larger set of capabilities.
This could manifest as a hang after altering the filters of a service at runtime.