<p>When there are two transactions that spend some of the same transaction outputs (TXOs), we call this “conflicting transactions”. Only one of the conflicting transactions can be included in the best chain since each TXO can only be spend once. If a transaction is malformed, spends non-existent funds, or is otherwise invalid, it can never be included in the best chain. Any block that would include such a transaction would be considered invalid and nodes follow the <em>valid</em> chain with the most accumulated work.</p>
<p>By the way, <a href="/questions/tagged/orphaned-blocks" class="post-tag" title="show questions tagged &#39;orphaned-blocks&#39;" aria-label="show questions tagged &#39;orphaned-blocks&#39;" rel="tag" aria-labelledby="tag-orphaned-blocks-tooltip-container">orphaned-blocks</a> are blocks that succeed a parent block that a node doesn’t know about. This phenomenon at least does not occur for Bitcoin Core nodes anymore since Bitcoin Core got the <em>headers-first synchronization</em> feature. (It may still be an issue for other node implementations, though.) Since the blockchain can only have one block at every height, whenever two blocks are found at the same height, one of them will not become part of the best chain. A node in such an extinct chaintip is usually referred to as <a href="/questions/tagged/stale-block" class="post-tag" title="show questions tagged &#39;stale-block&#39;" aria-label="show questions tagged &#39;stale-block&#39;" rel="tag" aria-labelledby="tag-stale-block-tooltip-container">stale-block</a>.</p>

CC BY-SA 4.0
2023-10-20T15:04:16.990Z
2023-10-20T15:04:16.990Z
Murch
<p>Invalid transactions are never included in valid blocks. They can be included in a block, but that block would be rejected as invalid (and not classified as an orphan block).</p>

CC BY-SA 4.0
2018-06-20T21:00:36.873Z
thanks. to finalise my understanding, are Orphan blocks just blocks that are valid but not placed on the longest chain (as the miners who made these orphan blocks) as the miner wasn't the first to solve the hash puzzle (POW)?
DCH
CC BY-SA 4.0
2018-06-20T22:06:26.220Z
Yes. Orphan blocks are completely valid, just not in the main chain.
Ava Chow
CC BY-SA 4.0
2018-06-18T19:39:58.743Z
2018-06-18T19:39:58.743Z
Ava Chow
<p>Do they get put in an orphaned block or not get put into any block at all?</p>

<transactions><transaction-verification><doublespend><block-validity>
What happens to rejected transactions (i.e double spend or invalid transactions)?
DCH
# What happens to rejected transactions (i.e double spend or invalid transactions)?

(Asked by: username_0 on 2018-06-18)

Do they get put in an orphaned block or not get put into any block at all?

**Tags:** transactions, transaction-verification, doublespend, block-validity


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## 2 answers

### Answer by username_2 on 2018-06-18. Score: 5 | Accepted answer

Invalid transactions are never included in valid blocks. They can be included in a block, but that block would be rejected as invalid (and not classified as an orphan block).

> **Comments:**
> **username_0**: thanks. to finalise my understanding, are Orphan blocks just blocks that are valid but not placed on the longest chain (as the miners who made these orphan blocks) as the miner wasn't the first to solve the hash puzzle (POW)?  
> **username_2**: Yes. Orphan blocks are completely valid, just not in the main chain.  

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### Answer by username_1 on 2023-10-20. Score: 0

When there are two transactions that spend some of the same transaction outputs (TXOs), we call this “conflicting transactions”. Only one of the conflicting transactions can be included in the best chain since each TXO can only be spend once. If a transaction is malformed, spends non-existent funds, or is otherwise invalid, it can never be included in the best chain. Any block that would include such a transaction would be considered invalid and nodes follow the *valid* chain with the most accumulated work.

By the way, [orphaned-blocks](/questions/tagged/orphaned-blocks "show questions tagged 'orphaned-blocks'") are blocks that succeed a parent block that a node doesn’t know about. This phenomenon at least does not occur for Bitcoin Core nodes anymore since Bitcoin Core got the *headers-first synchronization* feature. (It may still be an issue for other node implementations, though.) Since the blockchain can only have one block at every height, whenever two blocks are found at the same height, one of them will not become part of the best chain. A node in such an extinct chaintip is usually referred to as [stale-block](/questions/tagged/stale-block "show questions tagged 'stale-block'").


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