We got demonstrations earlier this year of how unrealized losses can quickly lead to failed banks. From Schiffgold at schiffgold.com:

Unrealized losses on securities held by US banks exploded by 22% in the third quarter.
Of course, unrealized losses don’t really matter — until they do.
This is yet more evidence that the financial crisis that kicked off last March continues to bubble under the surface.
Unrealized losses, primarily on US Treasuries and mortgage-backed securities rose by $126 billion in Q3 and now total $684 billion, according to the FDIC’s quarterly bank data release.

Current unrealized losses are only slightly below the record set in the third quarter of 2022. This reflects the fact that the FDIC took over three failed banks earlier his year and ate their unrealized losses when it sold the banks’ assets, thus wiping them from the books.
Unrealized losses on securities are divided between two accounting methods.
- Unrealized losses on held-to-maturity (HTM) securities jumped by $81 billion to $391 billion.
- Unrealized losses on available-for-sale (AFS) securities jumped by $45 billion to $293 billion.
It’s important to understand these are only paper losses. Ostensibly, the banks will hold these bonds until maturity and then will be paid their face value. If it plays out this way, there won’t be any real losses.