user104992: Some miners set the nVersion field of the block header to 0x8000004. What were they signalling for? Is this an error? I have been through a lot of the BIP documentations, but cannot seem to find any references for it.
The first one occurred at block height 416,832. The last at block height 455,757.
https://www.blockchain.com/btc/block/416832
https://www.blockchain.com/btc/block/455757

user3667: This block signaled support for BitPay's adaptive Block Size Proposal and was mined by slush pool.
I asked slush this exact question on twitter back in 2016!
https://twitter.com/slush_pool/status/744461234389524480
Note that this answer is not entirely complete, because the BitPay proposal specifies:

Miners express their support for this BIP by setting the
fifth-highest-bit in the block's 32-bit version number (0x08000000 in
hex)

But these blocks as you note also set bit 2 (0x00000004) which was not addressed by slush in his answer.
During this era, several other soft-fork proposals were being floated around that (due to lack of coordination a.k.a. decentralization) collided on their BIP9 signals. One was Extension Blocks. I'm having trouble finding links but I believe also at that time there was Rootstock (RSK) proposal that defined bit 2 as the signal, and an anti-asicboost proposal by Greg Maxwell that also signalled bit 2.

104992: <p>Some miners set the nVersion field of the block header to 0x8000004. What were they signalling for? Is this an error? I have been through a lot of the BIP documentations, but cannot seem to find any references for it.</p>
<p>The first one occurred at block height 416,832. The last at block height 455,757.</p>
<p><a href="https://www.blockchain.com/btc/block/416832" rel="nofollow noreferrer">https://www.blockchain.com/btc/block/416832</a></p>
<p><a href="https://www.blockchain.com/btc/block/455757" rel="nofollow noreferrer">https://www.blockchain.com/btc/block/455757</a></p>
 3667: <p>This block signaled support for <a href="https://github.com/bitpay/bips/blob/master/bip-adaptiveblocksize.mediawiki" rel="nofollow noreferrer">BitPay's adaptive Block Size Proposal</a> and was mined by slush pool.</p>
<p>I asked slush this exact question on twitter back in 2016!</p>
<p><a href="https://twitter.com/slush_pool/status/744461234389524480" rel="nofollow noreferrer">https://twitter.com/slush_pool/status/744461234389524480</a></p>
<p>Note that this answer is not entirely complete, because the BitPay proposal specifies:</p>
<blockquote>
<p>Miners express their support for this BIP by setting the
fifth-highest-bit in the block's 32-bit version number (<code>0x08000000</code> in
hex)</p>
</blockquote>
<p>But these blocks as you note also set bit 2 (<code>0x00000004</code>) which was not addressed by slush in his answer.</p>
<p>During this era, several other soft-fork proposals were being floated around that (due to lack of coordination a.k.a. decentralization) collided on their BIP9 signals. One was <a href="https://github.com/tothemoon-org/extension-blocks/blob/master/spec.md#activation" rel="nofollow noreferrer">Extension Blocks</a>. I'm having trouble finding links but I believe also at that time there was Rootstock (RSK) proposal that defined bit 2 as the signal, and an anti-asicboost proposal by Greg Maxwell that also signalled bit 2.</p>
