speaking of Bitcoin ... the july4th split

[coderman]

sometimes even consensus not enough,
 [ failure at 95% agreement! ]


SPV is a compensating measure that addresses block size without
reifying incompatibilities. however, with drawbacks and risk, as taken
on chin in this one:


https://bitcoin.org/en/alert/2015-07-04-spv-mining

"""
Summary: Some miners are currently generating invalid blocks. Almost
all software (besides Bitcoin Core 0.9.5 and later) will accept these
invalid blocks under certain conditions. The paragraphs that follow
explain the cause more throughly.

For several months, an increasing amount of mining hash rate has been
signaling its intent to begin enforcing BIP66 strict DER signatures.
As part of the BIP66 rules, once 950 of the last 1,000 blocks were
version 3 (v3) blocks, all upgraded miners would reject version 2 (v2)
blocks.

Early morning UTC on 4 July 2015, the 950/1000 (95%) threshold was
reached. Shortly thereafter, a small miner (part of the non-upgraded
5%) mined an invalid block--as was an expected occurrence.
Unfortunately, it turned out that roughly half the network hash rate
was mining without fully validating blocks (called SPV mining), and
built new blocks on top of that invalid block.

Note that the roughly 50% of the network that was SPV mining had
explicitly indicated that they would enforce the BIP66 rules. By not
doing so, several large miners have lost over $50,000 dollars worth of
mining income so far.

All software that assumes blocks are valid (because invalid blocks
cost miners money) is at risk of showing transactions as confirmed
when they really aren't. This particularly affects lightweight (SPV)
wallets and software such as old versions of Bitcoin Core which have
been downgraded to SPV-level security by the new BIP66 consensus
rules.

The immediate fix, which is well underway as of this writing, is to
get all miners off of SPV mining and back to full validation (at least
temporarily). As this progresses, we will reduce our current
recommendation of waiting 30 extra confirmations to a lower number.
"""