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Value overflow incident. I don't know what else of the technical facts you would love to know since the article explains it nicely and refer to the bug discussion thread.
When calculating the total value of transaction outputs, the software used a 32-bit integer variable that exceeded its maximum value due to a large output value.
Bitcoin did not use 32-bit integer values but rather 64-bit integer values all along.
The bug was fixed in Bitcoin Core version 0.3.10. The fix involved modifying the code to use a larger data type (64-bit integer) for handling transaction values, preventing similar overflow issues in the future.
The update did not change any data type, instead, it just added up some sanity checks [1].
[1]
https://github.com/bitcoin/bitcoin/commit/d4c6b90ca3f9b47adb1b2724a0c3514f80635c84#diff-118fcbaaba162ba17933c7893247df3aR1013