Hi Shayne,
As you know the Sequence Error is the generic "You did something I wasn't expecting" error - and of course what the register expects changes when the programming is altered.
I would carefully examine the PLU Status programming on the PLUs generating the error, looking particularly at "Preset", "Preset Override" and the "Disabled" settings closely.
On the ECR Keyboard, you can scan the PLU settings by using the following sequence:
Turn the mode lock to PGM mode (requires a key with "P" or "C" embossed on it)
Enter 15 SBTL
[First PLU with error]
[Last PLU with error]
CASH
Is your customer just pressing the PLU button? Maybe it is not Preset or it is disabled.
Is your customer entering a price before the item? Maybe Preset Override is not allowed.
I hope this helps. Let me know how you go.