Hey there! Retrofitting a 20-year-old Mitsubishi Q-series PLC with SIL-2 safety functions is a smart move, but you're right to be concerned about cascading failures. Here are the key practical considerations:

1. Physical Isolation is Critical: Don't try to integrate safety functions directly into your existing PLC logic. Mitsubishi offers safety PLC modules (like the QS series) that can work alongside your existing Q-series CPU. These have separate safety CPUs and function modules that operate independently.

2. Dedicated Safety I/O: Use separate safety-rated input and output modules. These have built-in diagnostics and redundancy that meet SIL-2 requirements. Your existing standard I/O should remain untouched for regular control functions.

3. Communication Boundaries: If you need to communicate between safety and standard systems, use safety-rated communication protocols with proper isolation. The safety system should only send status information to the standard PLC, not control commands.

4. Power Supply Separation: Safety systems need their own redundant power supplies. A failure in your standard system's power shouldn't compromise safety functions.

5. Risk Assessment First: Before implementing anything, do a proper risk assessment to identify what needs SIL-2 protection. Not everything needs safety-rated control - focus on critical safety functions only.

6. Documentation and Testing: Keep the safety logic simple and well-documented. Test extensively to ensure safety functions don't interfere with normal operation, and vice versa.

The key is maintaining clear separation between safety and standard control systems. This way, if your old PLC logic has issues, it won't cascade into the safety system, and safety system actions won't cause unexpected behavior in your existing control logic.