In short: I've made multiple mistakes in a calculation software within a year. These mistakes caused no harm whatsoever, but I'm afraid that I will lose credibility if I approach this the wrong way. I would really appreciate some advice on this issue, since this is my first job out of university.
The details: Since about two years I'm working in a larger company (about 100-200 people). Within the company I'm working in a subgroup of about 5 people. I'm solely responsible for maintenance and bug fixing of a 10 year old software (which is a huge mess). This software is used to compute safety margins on our products (e.g. calculating that a chair will hold up to 100kg). Last year someone reported a bug that some of the numbers calculated by the program were not adding up. I found the problem and it turns out, that the person was right. However, this was not an issue, since the problem was working in our favor (in the chair example, the allowed weight would be 105kg instead of 100kg). So no harm done, neither for the customer, nor for us. I presented this fix (among other things) during our yearly company wide internal update conference. So far, so good.
Today, I started to write a detailed report about the product we sold last year and took a closer look at the numbers. I noticed that something seemed slightly off, so I took a another deep dive into the program and found that the same bug, which caused the issues last year, was still present in a different part of the code and still caused the numbers to be slightly wrong (chair will hold 107kg would now be the correct result). Again, this causes no harm or problems for the company. However, different parts of the company rely on the calculated numbers and I will have to give a company wide update on the issue.
Now I'm afraid that this could cause my coworkers and the company to lose trust in me, the software package and the provided numbers. I'm not worried about losing my job, just about the credibility of my work. How would you approach this topic? Which point would you stress in the company wide update?