Encountering a Hash sum mismatch error during the execution of sudo apt-get update in Kali Linux can be attributed to various factors. These factors encompass network issues, repository problems, syncing errors, outdated repositories, package management system anomalies, package corruptions, software upgrades, disk issues, package verification discrepancies, and security measures such as proxy or firewall settings.

So, I encountered this error and I tried so many possible solutions. These included ensuring a stable internet connection, clearing the apt cache, temporarily disabling the firewall, and exploring other network-related adjustments.

I stumbled upon a solution, which helped solve the problem.
This was how I solved the issue:

Firstly, I paid a visit to Kali's official mirror site. I scrolled down and I saw something like in the image below.

I needed to view the closest mirror. And I scrolled down and saw this.

The closest to me seemed to be http://kali.download/kali/. So, I then decided to change my mirror link to it.

Now, the next question is, how did I view and change my mirror link?.
Firstly, run sudo su in your terminal to switch to the super user mode.
Then, run nano /etc/apt/sources.list to access the mirror link. You will see something like this

Change only the link in the second line to the nearest mirror link you got. As you can see, I already changed it.
Then run sudo apt-get update again.
I hope this helps.
Thanks.