Hey there! I've been through this exact transition myself, and let me tell you - there are definitely some sneaky surprises that only show up after the system goes live. Here are the most common 'gotchas' I've seen:

1. Timing issues that weren't obvious - Relays have physical delays, but PLCs execute in scan cycles. What worked perfectly with relay timing might fail with PLC logic because everything happens in milliseconds instead of the physical relay's mechanical response time.

2. Parallel vs sequential thinking - In relay logic, everything happens simultaneously. With PLCs, the scan cycle means operations happen sequentially. This can cause race conditions where the order of operations matters - something that never occurred with hardwired relays.

3. Hidden dependencies - Old relay systems often had undocumented 'features' where one circuit affected another through shared power supplies or ground loops. When you convert to PLC, these hidden relationships can break unexpectedly.

4. Software and hardware compatibility headaches - You'll often discover you need specific software versions, drivers, or cables that weren't part of the original plan. Getting the programming software to talk to the PLC can be a project in itself!

5. Electrical noise sensitivity - PLCs are much more sensitive to electrical noise than mechanical relays. What was 'good enough' wiring for relays might cause intermittent failures with PLCs due to noise interference.

6. Troubleshooting complexity - With relays, you could physically trace wires. With PLCs, you need the right software, laptop, and knowledge to see what's happening inside the program. This can turn a 5-minute relay fix into a 5-hour PLC debugging session.

7. Memory and backup issues - Unlike relays that just 'remember' their state through physical contacts, PLCs can lose their program if power fails or memory gets corrupted. Proper backup procedures become critical.

The biggest lesson? Always test thoroughly with the actual field devices connected, not just in simulation. And document everything - because you'll need to troubleshoot it someday!