When a number wraps around
Computers store integers within fixed ranges. If a calculation exceeds the maximum and software fails to check it, the result can wrap into a small or negative value. This is integer overflow.
In block 74,638, two outputs of roughly 92.2 billion BTC each pushed the sum outside the old numeric range. The flawed check failed to see the giant total and the old software accepted the block.
Fixing the rule and the chain
Users spotted the strange transaction on 15 August 2010. Satoshi and other developers prepared stricter range checks and released version 0.3.11. Nodes had to run the code and miners had to build on the branch without the exploit.
The repaired chain replaced the short invalid history. This was not a valid payment erased under unchanged rules; it was a correction to code that had failed to enforce the intended monetary rule.
What immutability means
A blockchain is not a file that can never change. Nodes choose a valid chain with the most accumulated proof of work, and short reorganisations are part of the design. More confirmations make replacement increasingly costly.
Open code and human review still matter. Bitcoin reduces reliance on one administrator but does not eliminate software bugs. Developers may propose code; independent node operators decide what to run.