When Lehman cratered, the support staff for their multiple, complicated trading programs went elsewhere, which makes unwinding the trades problematic.
This is made worse by the inevitable lack of documentation, both for users and programmers. I’m a database programmer, and going in on a contract to change a system you didn’t write that has no documentation, well, it can take weeks just to understand the code on a big system, much less start making changes.
But in Lehman’s case, there are probably billions of dollars of transactions in their 20-30 systems that no one quite knows how to operate any more. Yikes.