Certain RSpec patterns.

Subject should match what is being described

Instead of:
subject { instance }
before do
instance.update!(something: something)
expect(subject).to ...
It is clearer to handle the update in subject and then frame the assertion as a change.
subject { instance.update!(something: something) }
expect { subject }.to change { ... }

.to match vs. .to eq

Unlike in Jest, subset matchers only work with the looser .to match. When using Jest, expect.objectContaining, etc. can be used with both .toEqual and .toBe. However, a_hash_including, etc. will simply fail when used with .to eq! This is especially frustrating because nothing about the error message indicates that the failure comes from using the wrong matcher.

Diffing JSON blobs

RSpec inlines JSON blobs by default, which makes diffs very hard to detect. The super_diff gem helps with this by printing the diff line-by-line.