Hey there! As a purchasing director sourcing 'new old stock' (NOS) automation components, you're walking a tightrope between finding rare parts and avoiding costly compatibility nightmares. Here are the key red flags to watch for:

1. **Missing manufacturer support**: If the original manufacturer has stopped producing technical documentation, firmware updates, or replacement parts, that's a major warning sign. Check if the manufacturer still offers any support for that product line.

2. **Outdated communication protocols**: Older PLCs and automation components often use legacy protocols that don't play nice with modern industrial Ethernet or fieldbus systems. If the component uses proprietary or discontinued communication standards, integration will be challenging.

3. **No software compatibility**: Check if the programming software or configuration tools are still available and compatible with current operating systems. Many older automation components require outdated software that won't run on modern computers.

4. **Limited documentation**: If you can't find complete technical manuals, wiring diagrams, or programming guides, you're setting yourself up for installation headaches and potential system failures.

5. **Questionable storage conditions**: NOS components that haven't been properly stored can suffer from capacitor aging, corrosion, or other degradation issues that won't be immediately apparent.

6. **No testing capability**: If the seller can't provide proof of functionality testing, you're taking a big risk. Always ask for test reports or verification that the components work as intended.

7. **Single-source dependency**: If only one obscure supplier has the NOS components, you're creating a dangerous supply chain bottleneck. Look for multiple sources or consider alternative solutions.

8. **Missing certifications**: Older components may not meet current safety standards or regulatory requirements, which could create compliance issues for your operations.

The smart move is to balance maintaining critical spare parts inventory against obsolescence risks by developing a proactive obsolescence management plan. Consider modern alternatives or upgrade paths rather than relying on increasingly scarce legacy components. Remember, what seems like a bargain today could become a costly compatibility headache tomorrow!