I met someone who was still there 18 years later. IBM changed the way one type of variable was initialized when loaded, from zero to no change, so residual data remained and caused errors. They had to run emulators of prior machines and operating systems.
The app solved 40 equations with 90 unknowns. If you didn’t understand the math and didn’t have the design documentation, it would be impossible to update it.
They bought a replacement when desktop computers appeared, but it was not as accurate as mine. I suspect that they used single-precision floating point instead of double.
We had a small shop with projects for one to three people. Formal code reviews were not standard in the 60’s.
My last job, before retirement 4 years ago, didn’t do code reviews. They didn’t even have formal documentation or write user instructions. I added that for my projects and the users felt spoiled.