There are some events where bitcoins might form a negative value. For example if somebody stole a big amont of hardware illegimately and because of inability to identify to which bitcoins the illegit benefit where the whole pool of bitcoin might be fined with a ticket in conventional currency (that would be like the equivalent of saying that german franks are nazi money and anybody that is found in posession of it will be fined to have it confiscated). There is a high resistance to do that but since the analysis contains other portions with comparable uncertainty it starts to become relevant.