At least 25% of regular meditators have experienced adverse effects from mindfulness.

Evidence

How too much mindfulness can spike anxiety (BBC)

corroborated: 2 independent passages support this claim
XREF: connects to: Mindlessness leads to panic and detrimental behavior. — Dan Harris, 10 percent Happier, 40; Recent studies reveal that 25–87% of those who engage in meditation report adverse effects, with 3–37% experiencing functional impairment. — Adverse effects of meditation and mindfulness in clinical practice; Meditation and mindfulness, widely perceived as stress-reduction techniques, can also have adverse effects. — Meditation And Mindfulness May Lead To Anxiety, Other Adverse Effects In … (+3 more)

Recent studies reveal that 25–87% of those who engage in meditation report adverse effects, with 3–37% experiencing functional impairment. (similarity 0.95)
58% of 96 participants reported at least one meditation-related adverse effect, ranging from perpetual hypersensitivity to nightmares. (similarity 0.94)
Meditation and mindfulness, widely perceived as stress-reduction techniques, can also have adverse effects. (similarity 0.92)
The most common adverse effects are anxiety and depression, followed by psychotic or delusional symptoms and dissociation. (similarity 0.90)
Studies have described adverse events such as anxiety, pain, and psychosis associated with mindfulness meditation. (similarity 0.89)
About 30% of meditators experienced challenging or distressing effects, and 9% reported functional impairment. (similarity 0.88)
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 0.86)
A new harm monitoring study investigates the adverse effects of mindfulness-based practices. (similarity 0.85)
Harmful psychological treatments are damaging and cause worse outcomes than would have occurred without treatment. (similarity 0.83)