What have you suggested at work which sounded like a good idea at the time, but now sounds like a nightmare?

I inherited a nasty old legacy c# desktop app a few years ago, I was a sql developer so it was a steep learning curve, but I’ve tried to make it better, fixing things as I go.

I had the bright idea of mentioning that I would look at starting to add unit tests etc.

It seemed like a good idea at the time, but now I’m not so sure.

It depends. Unit tests require the code to be written in a particular way. If it’s not, you need to refactor it drastically just to make it testable. It will make the code better but it can be a huge amount of work. New bugs might come in the process but the unit test will hopefully find them.
Some bugs that you will introduce, you won’t be able to cover them with unit test. For example UI related bugs.
So, depending on the state of the code, it might actually be a bad idea.

@Lensflare yeah, my plan is to start with new stuff, and then gradually work back adding tests for existing code slowly overtime.

There isn’t a deadline for completion or anything like that, just doing bits as and when I can.

I fully expect that there will be large chunks where it just won’t be possible to do.

I suggested we do real agile/sprint planning with grooming and such. One big meeting at the end of the week to do it all.

It was perverted into meetings all week of the second week of the week. And our PM doesn't do the prior required work so we have to do all of that work in the meetings. I tried to add real process to end our nightmare and I just added to the nightmare.