Dred Scott, a 28 year old precedent overturned. 

The timeline:

1. Scott born into slavery → sold to Dr. Emerson (an Army surgeon, not a hunter)
2. Emerson takes Scott to free states/territories (Illinois and Wisconsin Territory) where Scott continues working as a slave without pay, even though slavery was illegal there
3. Emerson leaves for Louisiana but leaves the Scotts behind at Fort Snelling (still in free territory), where he leases them out to other officers—they’re generating income for Emerson while he’s gone
4. Emerson calls them back, and eventually they all return to Missouri (a slave state)
5. Emerson dies in 1843, and his widow Irene inherits the Scotts
6. Here’s the crucial moment: Scott tries to buy his freedom in 1846, offering $300. Irene Emerson refuses.
7. That’s when Scott sues, arguing “once free, always free”—because he lived in free territory for years, he should have gained his freedom there, and that freedom should stick even back in Missouri

The argument, once free always free, was compelling, the Missouri courts had upheld it for 28 years.

But suddenly everything changed.