Each time you save a change to a diagram, VIA validates the diagram's elements and their settings. If there are any problems, it will generate appropriate warnings or errors, and suggest "quick fix actions".
Warnings or errors show up as Error Markers in the Problems View below the main editor window, and as little error or warning icons above the diagram element:
Quick fix actions
For each generated error or warning marker, VIA tries to generate a suggestion to fix the problem. Click the error or warning icon above the diagram element to display a popup menu of suggestions: