The money was in limbo due to a bug in a contract. There are contracts (monsters) that scan for such bugs and situations to profit from them. The guy tried to save the money from the monsters and failed.

The situation was worse since it happened already in the mempool where the pending transactions reside.

I am dabbling a bit in the Tezos currency and environment and find this 2 language projects interesting aimed to increase safety: https://github.com/metastatedev/juvix (alpha) and https://archetype-lang.org/ Not saying it might have helped in the concrete case.