Shapiro (1992) observed the effects of vipassana meditation on a small group of meditators and found that while most participants experienced positive results, a small number of meditators experienced distinctly negative states.
Glueck (1984) studied 110 participants and reported that the practice of transcendental meditation (TM) appeared to release repressed subconscious impressions. A small proportion of participants reacted adversely to this experience. Heide (1983, 1984) found that 54% of anxiety prone participants demonstrated increased anxiety during mantra meditation modelled on TM. Otis (1974) observed a cohort of 62 novices who tried TM and concluded that it was not suitable for those with serious emotional problems.
Read more about the research into the adverse effects of meditation here.