user65745: Is it possible to have a "unlucky" transaction that can never be part of block? Meaning grouping it with other transactions never results in a valid block?
[Edit]
By not being in valid block, I meant no valid nonce is found with given target difficulty. It seems mathematically, this is possible and what happens to transaction (if there is no expirey) in this case?

user12983: No. By the definition, any valid transaction can be placed in block. If something in block invalidates it - this is only an array of random bytes, not a valid bitcoin transaction.

user64508: If you mean that the transaction may be pending for hours, or even days, yes: it can happen and it's part of Bitcoin Core protocol.
You may also want to have a look at this question

user51948: No, if the transaction is valid then it can always be included in a block if a valid nonce/extra nonce is found.

65745: <p>Is it possible to have a "unlucky" transaction that can never be part of block? Meaning grouping it with other transactions never results in a valid block?</p>

<p>[Edit]
By not being in valid block, I meant no valid nonce is found with given target difficulty. It seems mathematically, this is possible and what happens to transaction (if there is no expirey) in this case?</p>
 12983: <p>No. By the definition, any valid transaction can be placed in block. If something in block invalidates it - this is only an array of random bytes, not a valid bitcoin transaction.</p>
 64508: <p>If you mean that the transaction may be pending for hours, or even days, yes: <a href="https://en.bitcoin.it/wiki/Transaction_expiration" rel="nofollow noreferrer">it can happen</a> and it's part of Bitcoin Core protocol.</p>

<p>You may also want to have a look at <a href="https://bitcoin.stackexchange.com/questions/53852/how-to-unstuck-a-low-fee-transaction-with-blockchain-info">this question</a></p>
 51948: <p>No, if the transaction is valid then it can always be included in a block if a valid nonce/extra nonce is found.</p>
