Its interesting and hard to comprehend but it is true, a company can have negative BV. Look at GM for example, they were trading for the longest time with a negative BV. A lot of “zombie banks” in Japan have negative BVs. It just means that they have sustained net income losses over time which has eaten completely into their OE, doesnt mean that the company is insolvent.
Another way, say you have a MASSIVE investment portfolio like an insurance company with assets all held as available for sale, say your assets/equity is 10x, if your portfolio goes down 10%, then your equity is wiped out through negative aoci.
Anyways, just wanted to share… I asked the same question a while back and studied it deep.