When capital ships outgrew armour against their own guns

What was the first capital ship not armoured against its own weapons?

The panel tentatively identifies Invincible among purpose-built capital ships, while noting classification and later gun changes complicate the answer.

Invincible is the panel's tentative answer among purpose-built capital ships. Japanese Tsukuba-class armoured cruisers with 12-inch guns complicate the chronology, but their original cruiser classification makes them a different case. Agincourt is also raised because of her light protection, though the discussion does not establish that her armour failed the exact test posed. The North Carolina class illustrates another route to the problem because ships initially protected for 14-inch guns received 16-inch batteries late in development.