[grrowl]

It only takes one break in the chain to compromise the entire web of trust. With such wide-spread connections across the planet these days, the chance of someone you trust three times removed accidentally breaking that chain is quite real.

[FreeFull]

Let's say you see some spam. You could have the software tell you which part of the web has made the spam trusted, and then you could manually mark that part of the web as untrusted. If there are only a few breaks in the chain like that, it'd be a workable solution.