I work at a tech consulting company in the US and I recently took over a project from a coworker (Bob) who got laid off. (He was mainly let go because of covid, but he wasn't the easiest to work with and people occasionally had complaints about him.)
A few weeks before Bob left, he deployed a big feature into production. The deployment seemed to work properly. He did not know at the time that he would be laid off and there was no particular extra pressure/stress involved with the deployment.
The client asked me last week about an issue they were seeing with the production, where some of the data wasn't flowing through. Nothing major but it raised quite a few eyebrows from the client side.
After looking into it, I see that Bob had forgotten to remove a testing filter which was limiting the input data. This means that for the last month the process wasn't working for a significant section of the target data. Thankfully it is fixable without permanent damage.
I honestly sympathize with the client about this: It was quite a big oversight. How can I professionally acknowledge that my former colleague made this mistake and express that I'm also not-happy about this, but without making a big deal out of it or sounding like I'm bad mouthing Bob?