What happens if T cells don't undergo T cell selection?

If T cells do not undergo positive selection, non-functional T cells that are not capable of interacting with MHC (Major Histocompatibility Complex) molecules and hence useless to the immune system would enter the blood stream along with functional T cells. This crowding out of functional T cells slows down the rate at which they can form appropriate immune responses.  

If T cells do not undergo negative selection, autoreactive T cells that mistakenly recognize healthy tissues in the body as foreign pathogens would enter the blood stream, resulting in autoimmune disorders. 

Positive selection and negative selection are both equally important for the immune system to function efficiently and defend the body against harmful infections while preventing attacks on its own tissues.