This is an unfortunate accident LL, but I can´t see that it´s the fault of either set of guests in any way. The drawer was faulty - if the guest with the damaged toe had decided to make a claim against you then I suspect he´d have a case. It would indeed have been better if the first set of guests had informed you about the dodgy drawer, but they did what they thought was sensible and can´t be said the have been at fault - they were lucky that it did not collapse on them and cause injury.
Of course you´re right - you can´t fix something you don´t know about, but that is always the case. Ultimately we are responsible for condition of our properties.