A shared plan needs room for the person who understood it differently.

Shared plans are held in two heads, and the two versions are never identical. Somebody heard an emphasis differently, or filled a gap with a reasonable assumption, and the difference stays invisible until the work meets. Treating the mismatch as carelessness misses that both people were being sensible with what they had at the time.

Before starting, ask each person to say back what they think happens next and by when. When the versions differ, correct the plan rather than the person. Leave slack in the schedule for one round of realigning, because there will usually be one.