How can I prevent this error from happening again? I had set a max bid of 167.57 on this item and it failed to bid. The item then sold for 100.

Hello Jeff,
Thank you for your question about this snipe. That error means two things,
1. The error isn't programmed into our system yet.
2. eBay didn't report it as a win.
If you click your log icon, the error is revealed as, "The quantity was either missing, invalid, or zero."
This is error 79 on this page: https://developer.ebay.com/Devzone/xml/docs/Reference/ebay/Errors/ErrorMessages.htm .
As far as why eBay returned this, I have no idea, as we always submit a quantity of one (1). Here is exactly what we sent:
=====
User auth: [redacted]
ItemID: 186331875470
Action=Bid
Amount=167.57
Quantity=1
UserConsent=true
=====
I wish I had more info, but by providing the input, output, and detailed log of every action, there literally isn't anything else we can do. Frankly, it looks like a fluke. I've never seen this in 20 years.
We're going to program this error in so if it happens again, we will know it's a repeat.