Experiments are isolated — one active experiment per contact at a time, or interaction effects are handled

When a contact is simultaneously enrolled in a subject-line test and a send-time test that both use open rate as the outcome metric, the results of each experiment are contaminated by the other. You cannot isolate the effect of the subject line from the effect of the send time. Reported lifts from both experiments are artifacts of the interaction, not the individual treatments. In practice this leads to shipping changes that performed well in a contaminated test but have no effect or a negative effect in production. The data-integrity failure is structural: the experiment framework appears to produce results, but those results are not attributable to the variable being tested.

Implement exclusion groups so contacts enrolled in one experiment sharing a metric are blocked from enrollment in another. Check at assignment time, not at analysis time:

async function canEnrollContact(
  contactId: string,
  experimentId: string
): Promise<boolean> {
  const experiment = await db.experiments.findUnique({ where: { id: experimentId } })
  if (!experiment.exclusion_group) return true

  const conflict = await db.experimentAssignments.findFirst({
    where: {
      contact_id: contactId,
      experiment: {
        exclusion_group: experiment.exclusion_group,
        status: 'active',
        id: { not: experimentId }
      }
    }
  })

  return conflict === null
}

Tag every experiment with an exclusion_group string ('open-rate', 'click-rate', etc.) at creation time. Experiments testing independent levers (e.g., subject line vs. footer copy) can share a group if they share the primary outcome metric.