Hey there! I've been digging into PLC-5 to ControlLogix migrations, and let me tell you - this is where the real engineering nightmares hide! Based on what I'm seeing, here are the biggest hidden pitfalls that can absolutely wreck your cutover:

1. Indirect Addressing Landmines - PLC-5's indirect addressing syntax (like N[N251_0][1]) doesn't translate cleanly to ControlLogix. The conversion tools often choke on these, creating tags that look right but fail at runtime. I've seen cases where the translate tool creates tags that just return errors during execution.

2. Memory File Mapping Gotchas - When the conversion tool creates base tags like "B3" to mimic PLC-5 data files, the HMI mappings can get completely lost. Proficy iFix and other HMIs sometimes can't write to integer bits in ControlLogix after migration because the memory mapping isn't 1:1.

3. Symbol-to-Alias Confusion - The conversion tool takes RSLogix 5 symbols and creates Alias tags, but these don't always behave like the original PLC-5 memory addresses. HMI systems expecting direct file/word/bit addressing get completely confused.

4. Controller Scope Tag Issues - Rockwell's own documentation shows that tags must be controller scope to map properly to PLC-5/SLC files. If your migrated tags aren't scoped correctly, the mapping just fails silently.

The real killer? These issues often don't show up until cutover when the system goes live. One engineer mentioned their plant had PLC-5 code "very heavy in its use of indirect addressing" - exactly the kind of legacy code that migration tools struggle with most.

Have you run into any of these specific issues, or are you dealing with a particularly complex PLC-5 system?