During the planning for activation of the mid-split spectrum on the cable plant, adjacent channel interference (ACI) between the mid-split upstream spectrum and legacy devices on the network that use the same spectrum on the downstream was anticipated. Extensive testing was conducted on DOCSIS® and set-top devices to understand the impact on the performance of these devices and the customer-impacting thresholds. Based on this testing, tools have been developed to identify this interference and prevent negative customer experiences when the mid-split spectrum is activated. With millions of mid-split activations in the field, we have been able to see how well the tools that look for interference perform. We also have gained better insight into how ACI impacts the RF front end and performance of the CPE devices and how in-home wiring can have a large impact on ACI. As the industry moves to DOCSIS 4.0 technology, ACI involves not just co-located in-home devices but also neighboring devices. In this paper, we will review our current prediction tools for ACI, field and lab test results of CPE devices in the presence of ACI, and improvements which have been made to the prediction tools. Most important will be how these tools and lessons learned can be adapted to understand and predict neighbor interference to ensure an optimal customer experience as the upstream spectrum is expanded further in DOCSIS 4.0 technology.