If you are referring to something we call "ghosting" where lines or parts of a slide bleed through to become visible on later slides, then the solutions usually are:
- Change slide quality settings to Optimized or High (24bit)
- Turn off compression settings in Preferences > Project > SWF Size and Quality.
Once you have rectified the issue, try turning compression settings on again until you find the one causing the issue. The most likely candidate is Advanced Project Compression.
