Transparency and overprint can look correct on screen but produce wrong color or knockout in print. Flattening and overprint simulation need to be handled before the file goes to press.
Transparency issues
Complex transparency (blend modes, opacity stacks) can flatten differently in the RIP than in your design app. Banding, color shift, or unexpected knockouts can appear. Simplify or flatten in a controlled way before export.
Overprint
Overprint means one color prints on top of another instead of knocking out. Used intentionally for trapping or rich blacks; used by mistake, it can make type or graphics disappear or change color. Check overprint preview before exporting.
Before you export
• Use Overprint Preview (or equivalent) to see how overprints will print.
• Flatten transparency in a way that preserves the look you want.
• If in doubt, export to PDF/X-4 or ask the printer how they want transparency handled.
Common mistake
Sending a file with accidental overprint on black type. On press, the type can disappear or print as a muddy mix. Always check overprint settings before sending.