So, we have 3 main units.

Unit C has the most experienced workers - easiest work, but quite critical, and the machinery requires more continual observation.

Everyone in all units knows how to do all of the jobs, but we realized, this morning, that we aren’t…. Doing updates, I guess.

Some people from Unit B filled in for Unit C, and made a mistake which was made years ago.

(Not terribly critical. Some packaging was formed “borderline” in spec.)

A full retraining won’t work.

Has anyone experienced a system which they felt did a good job of saying “don’t do X. It seems like a good idea, but it doesn’t work.”)

What's creating the idea of doing X?

Team C had the idea, tried it. It didn’t work, so they didn’t do it again.

Team B rotated in to help fill temporary openings in Team C, thought of the idea as well, and also tried it.

Team C tried it, at first, between when Team B was trained on the C equipment, and when Team B filled some slots.

I have always worked with some type of turnover logs and a management of change system. Both were used to pass on information and most tasks has written procures that must be followed.

That is on top of a formal training system where all employees go through the same training. We always have brief daily safety meetings and more extensive monthly meetings. Those are all used to keep everyone informed.

Do the heads of the teams (management, sup0ervisors, etc) have regular staff meetings? A lessons-learned segment in those meetings would be helpful.

Basically, you need some overlap from Team C to Team B if B is going to be doing C's job. Perhaps it is an actual C member being there but it could be that C's info gets passed to B's leadership in meetings or written down somewhere. That way you don't have to retrain the entire team.

When I was working, we used a Knowledgebase App.

Populate it with things like this and in the future one just needs to do a lookup with words that are key to describing the issue or the proposed solution.
A hit would come up with all the information that pertained to the problem. You would be able to include the "don't do this" instructions in the entry.

I can't recommend an app because we used one that I had written for the company.
They are available, though; maybe even free on the Open source domains.

We (engineers) have a monthly "lunch and learn" that one of us runs. The company buys lunch and we disuss the topic of the day. Its a good way to have an informal meeting of the minds.

Sometimes its a senior engineer going over something that we should all know, but its a kind of a reset to make sure everyone is performing the work in a similar fashion. Other times, we task a junior engineering tech to present a topic to get them to learn how to self learn and present information.

We do lessons learned from in-house case studies, discuss procedures and techniques for the work we do to try to see if we can make it more efficient and safer, we present new technologies that might influence how we do business, and we go over basic fundamentals. Sometimes, we bring a vendor in to educate us their products.

These meetings are helpful to keep everyone on the same page.

Get a composition book and a pen and have each team write things down they think will help if new people had to fill in..then you hand it to them and let them fill it up with knowledge. Doesn’t have to be in any order as they submit it. Later you read it and separate the info to appropriate sections.

Document, document, oh, did I say document?

Every machine and process should have a manual.

No one gets to play without reading it.

I'm a fan of procedures manuals.

In your case, it sounds like you need a Maintenance and Troubleshooting Manual.

It would be nice if everyone who is going to do maintenance will bring the book to where they're doing whatever process they'll be doing and follow the steps as they go along.

At the very least, have them review the procedure as part of the process.

The only ones who fuck it up are those who think they know it all but don't.

I thought the MOC process was required by OSHA 1910. I guess the extent to which it is implemented is not regulated. A good PSM process usually solves most of those issues.