1. Duplicate txid coinbase bug → BIP-30
Blocks 91,842 and 91,880.
Two coinbase transactions reused the same txid as earlier coinbases. Later ones overwrote earlier ones in the tx index. When BIP-30 locked in the rule “no duplicate txids,” the earlier outputs became permanently unreachable.
Coins were not reassigned. They were economically burned by rule finalization.
This is the cleanest example of what you’re pointing at.
2. 2010 value overflow bug → emergency reorg
Block 74,638 (August 2010).
A transaction exploited an integer overflow and created ~184 billion BTC. That block was later invalidated via a coordinated reorg and patched client release.
What happened economically:
• Outputs created in that block ceased to exist.
• Coins “spent” from that block were undone.
• The supply was corrected by deleting history.
This is outright confiscation via rollback, even if justified. It’s the strongest counterexample to “Bitcoin can’t take coins.”
3. Script rules tightened via soft forks (edge cases)
BIP-66 (strict DER signatures), BIP-62 groundwork, later script tightening.
These soft forks made some scripts that were previously acceptable under loose rules no longer valid.
In practice:
• Very few real outputs were affected.
• Any affected outputs became unspendable unless already spent.
This is rare and mostly theoretical, but it fits the definition: a rule change removed spendability without moving coins.
4. OP_NOP repurposing (e.g., BIP-65 / CLTV)
OP_NOP2 → CHECKLOCKTIMEVERIFY.
Scripts that relied on OP_NOP2 doing “nothing” could, in theory, break. Any such outputs would become unspendable post-fork.
• Practically nonexistent usage.
• Conceptually real: semantics changed, spendability changed.
Not confiscation in spirit, but yes in mechanism.
5. Anyone-can-spend outputs via invalidated blocks
Whenever blocks are later ruled invalid by tightened consensus (rare but nonzero), outputs from those blocks disappear permanently.
This happened in small incidents (e.g., early Berkeley DB lock limit fork in 2013), though coins were usually recovered via reorg. If they hadn’t been, they’d qualify.