This might be a bit of a random question, but there must be many developers who have some experience of code that broke after an upgrade. I am in particular looking for breaking changes when upgrading from Web 8.1 to Web 8.5.
Actually I have quite the opposite experience, when developing DXA we didn't need to change/fix anything when upgrading from Web 8 to Web 8.5.
There are backwards compatibility breaks and API changes in the 8.5 release of course, they are fully documented here. Some highlights which might be valid for you could be:
- User privileges no longer identical to Access Token privileges
- Work Items contains dynamic versions, not versionless items
- Undo publish feature dropped