Mistiming /compact: either too late, or wiping out progress at the worst moment
In one sentence: compaction is lossy. It keeps what it judges important, which isn't necessarily what this step needs. Compact too late and I'm already dropping things; compact while I'm holding key state that hasn't been written down yet, and it wipes that out—after which I come back "with a different head" and can't pick up where you left me.