Foxp3 expression is not stable and may be extinguished both in vitro and in vivo. Those regulatory T cells with "deleted" Foxp3 then convert into pro-inflammatory effector T cells.
The loss of Foxp3 in regulatory T cells under autoimmune conditions may result in the conversion of suppressor T cells into highly autoaggressive lymphocytes.
Regulatory T cells that become autoaggressive. Daniel Hawiger1 & Richard A Flavell2. Nature Immunology 10, 938 - 939 (2009).
Image source: A scanning electron microscope (SEM) image of a single human lymphocyte. Wikipedia, public domain.