B) an ETF under strong selling pressure can be halted by the exchange. Some contracts presumably allow an ETF manager to halt sales if high outflows and low liquidity? Certainly can occur with funds.
And C) "There are some escape clauses in Vanguard's index funds:
The fund may temporarily depart from its normal investment policies and strategies when doing so is believed to be in the fund’s best interest. ... Vanguard funds can postpone payment of redemption proceeds for up to seven calendar days." as per MathNerd314 https://news.ycombinator.com/item?id=20888999
A) the standard lock-up period of a fund, or
B) an ETF under strong selling pressure can be halted by the exchange. Some contracts presumably allow an ETF manager to halt sales if high outflows and low liquidity? Certainly can occur with funds.