What valuable knowledge do humans repeatedly rediscover because we fail to preserve it?

A few patterns show up again and again, where knowledge gets lost and painfully relearned:

Practical and technical

• Materials and processes. Roman concrete's durability, certain historical steel and glass-making techniques, and specific dye or pigment recipes have been lost and partially reconstructed. Industrial know-how often lives in people, not documents, so it evaporates when a factory closes or a generation retires.
• Institutional memory in organizations. Companies and governments repeatedly rebuild the same tools, hit the same failure modes, and relearn why an old rule existed—often after removing it (the "Chesterton's fence" problem).

Health and daily life

• Preventive basics. Handwashing and sanitation were championed (Semmelweis, others), resisted, and had to be re-established. Nutritional lessons like the cause of scurvy were discovered, forgotten, and rediscovered more than once.
• Traditional ecological knowledge. Local practices around soil, water, fire management, and which plants are useful or toxic are often dismissed, lost, then rediscovered by science later.

Social and organizational

• Why safeguards exist. Financial crises, public-health failures, and engineering disasters frequently repeat because the reasoning behind past regulations gets forgotten or deliberately dismantled.
• Craft and tacit skills. Skills passed by apprenticeship—navigation without instruments, traditional construction, artisanal techniques—vanish when the chain of teaching breaks.

Why this keeps happening

• Much valuable knowledge is tacit (held in hands and habits, hard to write down).
• Records decay, formats become unreadable, or context is lost even when data survives.
• Incentives reward novelty over maintenance and documentation.
• Generational and cultural discontinuities (disruption, migration, colonization) sever transmission.

If you have a particular domain in mind—engineering, medicine, ecology, organizations—I can go deeper.