Vue, a program for modeling and rendering landscapes, was very highly recommended to me, but it was full of bugs. Bringing these bugs up with customer support and telling them about how to repro the bugs got me the reply "Oh, well, no one would do that."
S'cuse me? No one would use a sculpting brush after applying a height filter in the terrain editor? That's a pretty common practice, and I should know. Most of my classmates did that with Bryce in college. Bryce wasn't bug-free either but at least we never got fed bullshit about what common tools people will and won't use together.
Deeply regretted that purchase, especially since it was a $1,000 program.
Cinema 4D, I've gotten more use out of by far, but I also feel a lot of regret about giving Maxon my money for it. Not because I miss the money, but because I feel I rewarded people for not caring at all about how well they did their job. I've been using it for years and I still find new bugs almost every day that cost me anywhere from 5 minutes to 5 hour's worth of creating workarounds. There was a single free patch, but I had to uninstall it because the patch broke a set of tools that I couldn't find an alternative for.
This version is old, and while I would hope that newer versions have improved, I'm not paying another couple thousand dollars to a company that does this, releases a single feature-breaking patch, and then requires you to pay for any further upgrades. I feel like I'd be degrading myself by doing so; suggesting that I'm fit only to open my wallet and gobble up whatever scraps they deign to sweep off the table.
Those are the two greatest combinations of expense and regret that I have.
