The most common adverse effects are anxiety and depression, followed by psychotic or delusional symptoms and dissociation.

Evidence

Meditation can be harmful – and can even make mental health problems worse (The Conversation)

corroborated: 2 independent passages support this claim
XREF: connects to: Mindlessness leads to panic and detrimental behavior. — Dan Harris, 10 percent Happier, 40; Studies have described adverse events such as anxiety, pain, and psychosis associated with mindfulness meditation. — What Are Adverse Events in Mindfulness Meditation? - PMC - NIH

Studies have described adverse events such as anxiety, pain, and psychosis associated with mindfulness meditation. (similarity 0.93)
At least 25% of regular meditators have experienced adverse effects from mindfulness. (similarity 0.90)
58% of 96 participants reported at least one meditation-related adverse effect, ranging from perpetual hypersensitivity to nightmares. (similarity 0.89)
Recent studies reveal that 25–87% of those who engage in meditation report adverse effects, with 3–37% experiencing functional impairment. (similarity 0.88)
Meditation and mindfulness, widely perceived as stress-reduction techniques, can also have adverse effects. (similarity 0.87)
Meditation practice in MBPs is associated with transient distress and negative impacts at similar rates to other psychological treatments. (similarity 0.87)
Anxiety, traumatic re-experiencing, and emotional sensitivity were the most common MRAE; 10.6% reported some functional impairment. (similarity 0.83)
Harmful psychological treatments are damaging and cause worse outcomes than would have occurred without treatment. (similarity 0.83)
About 30% of meditators experienced challenging or distressing effects, and 9% reported functional impairment. (similarity 0.83)
A new harm monitoring study investigates the adverse effects of mindfulness-based practices. (similarity 0.81)