Koha should return useful screen messages for patrons when a checkout fails for some reason, currently it seems we just get the last kye/value pair:

In C4 / SIP / ILS / Transaction / Checkout.pm

Line 69 is quite correct:

  67     if (scalar keys %$issuingimpossible) {
  68         foreach (keys %$issuingimpossible) {
  69             # do something here so we pass these errors
  70             $self->screen_msg($_ . ': ' . $issuingimpossible->{$_});
  71             $noerror = 0;
  72         }
  73     }