Hey there! As someone who's made the jump from relay logic to PLC programming, I totally get where you're coming from. The visual similarity between ladder logic and relay diagrams can be deceiving - it looks familiar, but there are some really counterintuitive concepts that trip up beginners. Here are the biggest ones:

1. **Parallel vs. Procedural Thinking**: This is the biggest mind-bender! In relay logic, everything happens simultaneously in hardware. But in ladder logic, the PLC scans your program sequentially from top to bottom, left to right. Experienced programmers know that the order of rungs matters - what happens on rung 10 can affect what happens on rung 20 in the same scan cycle. Beginners often think everything happens at once like physical relays.

2. **The Scan Cycle Trap**: Related to the first point, the entire program executes in a loop called a scan cycle. Inputs are read once at the beginning, logic is processed, then outputs are updated. This means if you change an output in one rung, it won't affect contacts using that same output until the NEXT scan cycle. This creates timing issues that don't exist in physical relay systems.

3. **Memory Types Confusion**: In relay logic, you have physical contacts and coils. In ladder logic, you have bits, words, integers, timers, counters - all stored in different memory types. Beginners often struggle with when to use internal bits vs. actual I/O, or how data types affect math operations.

4. **Timers and Counters Behavior**: These work completely differently than their physical counterparts. A timer in ladder logic might not behave exactly like a physical timer relay - they're software constructs that depend on scan time and can have different reset behaviors.

5. **Contact vs. Coil Misunderstanding**: In relay logic, a contact is a physical switch. In ladder logic, a contact (like XIC - Examine If Closed) is actually checking the STATUS of a bit in memory. This subtle difference leads to confusion about what's being examined versus what's being controlled.

6. **Race Conditions**: Because of the sequential scan, you can create race conditions where the order of execution creates unexpected results. Two rungs that seem like they should work simultaneously might interfere with each other in ways that don't happen with physical relays.

The key insight that experienced programmers have is that ladder logic is SOFTWARE that LOOKS LIKE hardware. Once you start thinking in terms of scan cycles, memory states, and sequential execution rather than physical circuits, these concepts become much clearer. It's a different mindset that takes time to develop, but you'll get there!