Short answer: when the garment color bleeds through to the transfer.
Dye Migration (also known as color bleed) is a rare issue that can occur usually on 100% Polyester or Dye-Sublimated garments in which the color of the garment may bleed through and effect the transfer color. If your garments dye is not set properly or was not properly cured, the heat of transfer application can cause the dyes to react and resurface either immediately or sometimes after a couple of days.
Fortunately, many of our formulas include dye blocking properties to help prevent this, including our Process + Blocker Heat Transfers. If you are concerned about possible dye migration, you can order samples to test with here
Or if you have any questions, please contact our support team!