I ran a certain ODE in April 2019 and got a certain result. The code I used and the results were saved using git for version control. I ran the same ODE now in October, with the same code which I pull from the repository, and the integration diverges after ~3% of the total time has been simulated.
The computer is the same (it has not been formatted, although I did install an Ubuntu partition on it), the MATLAB version is the same (R2017b) and the code is the same. Now, the ODE is somewhat ill-conditioned and I had experienced divergence before when modifying slightly some of its parameters, however I am sure from the version control that I am running exactly the same code as before.
I know that the most probable reason is that I am not running the same code, but I really cannot find the difference. Are there any other reasons this might be happening?