When the money is needed soon or the fund is the whole portfolio. Counterexample, on the same daily data: $100,000 put into QQQ at its peak on Mar 24, 2000 was worth $17,019 at the Oct 9, 2002 low; the same amount in SPY was worth $52,495. The QQQ holder needed until Feb 2015 to get back to $100,000, almost 15 years, without withdrawing anything. Someone who retired in 2000 and took withdrawals from that balance would have been further behind still, because shares sold near the low never come back.
The reverse mistake also exists: a 30-year-old who sells QQQ after a 35% fall turns a paper loss into a realized one and misses the recovery the tilt was supposed to ride out. Morningstar found that the gap between what funds return and what their investors earn is wider in volatile funds (Morningstar). If you would not hold through 2022 again, the tilt is too big.