What are the negative effects of meditation? I see lots of articles touting the benefits of meditation. But what are the bad things about it? Maybe if a person has to make quick decisions, meditation would not be a good thing? This could be similar to a paridigm of "Thinking Fast, Thinking Slow" by Kahneman.
The adverse effects of meditation as reported in scientific studies are as follows:


relaxation-induced anxiety and panic
paradoxical increases in tension
less motivation in life
boredom
pain
impaired reality testing
confusion and disorientation
feeling 'spaced out'
depression
increased negativity
being more judgmental
feeling addicted to meditation
uncomfortable kinaesthetic sensations
mild dissociation
feelings of guilt
psychosis-like symptoms
grandiosity
elation
destructive behavior
suicidal feelings
defenselessness
fear
anger
apprehension
despair
increased false-memory susceptibility


Also, anecdotal evidence suggests that meditation can worsen symptoms of some psychiatric problems.

Sources:

Study from 1992:
http://www.ncbi.nlm.nih.gov/pubmed/1428622


  Adverse effects of meditation: a preliminary investigation of
  long-term meditators, by Shapiro DH Jr.
  
  Department of Psychiatry and Human Behavior, California College of Medicine, University of California, Irvine.
  
  Abstract
  
  Adverse effects of meditation were assessed in twenty-seven long term
  meditators (average 4.27 years) both retrospectively (time one) and
  prospectively at one month (time two) and six months (time three)
  following a meditation retreat. At both time one and time three
  subjects reported significantly more positive effects than negative
  from meditation. However, of the twenty-seven subjects, seventeen
  (62.9%) reported at least one adverse effect, and two (7.4%) suffered
  profound adverse effects. When subjects at time one were divided into
  three groups based on length of practice (16.7 months; 47.1 months;
  105 months) there were no significant differences in adverse effects.
  How the data should be interpreted, and their implications both for
  the clinical and psychotherapeutic use of meditation as a
  relaxation/self-control strategy, and as a technique for facilitating
  personal and spiritual growth, are discussed. Limitations of the study
  and suggestions for future research are also offered.


Meta-analysis from 2000:


  Meditation: concepts, effects and uses in therapy by Alberto
  Perez-De-Albeniz and Jeremy Holmes
  
  International Journal of Psychotherapy, Mar2000, Vol. 5 Issue 1,
  p49, 10p
  
  Abstract: This article reviews 75 scientific selected articles in the
  field of meditation, including Transcendental Meditation among others.
  It summarizes definitions of meditation, psychological and physiological
  changes, and negative side-effects encountered by 62.9% of meditators
  studied. While the authors did not restrict their study to TM, the
  side-effects reported were similar to those found in the "German
  Study" of Transcendental Meditators: relaxation-induced anxiety and
  panic; paradoxical increases in tension; less motivation in life;
  boredom; pain; impaired reality testing; confusion and disorientation;
  feeling 'spaced out'; depression; increased negativity; being more
  judgmental; feeling addicted to meditation; uncomfortable kinaesthetic
  sensations; mild dissociation; feelings of guilt; psychosis-like
  symptoms; grandiosity; elation; destructive behavior; suicidal
  feelings; defenselessness; fear; anger; apprehension; and despair.


New study shows increase in cortisol reactivity (a biological marker of stress): http://www.ncbi.nlm.nih.gov/pubmed/24767614


  The present study provides an initial indication that brief
  mindfulness meditation training buffers self-reported psychological
  stress reactivity, but also increases cortisol reactivity to social
  evaluative stress. This pattern may indicate that initially brief
  mindfulness meditation training fosters greater active coping efforts,
  resulting in reduced psychological stress appraisals and greater
  cortisol reactivity during social evaluative stressors.


Another new study shows increased susceptibility to false memories:

http://pss.sagepub.com/content/26/10/1567

Increased False-Memory Susceptibility After Mindfulness Meditation by Brent M. Wilson, Laura Mickes, Stephanie Stolarz-Fantino, Matthew Evrard and Edmund Fantino in Psychololgical Science


  Abstract
  
  The effect of mindfulness meditation on false-memory susceptibility
  was examined in three experiments. Because mindfulness meditation
  encourages judgment-free thoughts and feelings, we predicted that
  participants in the mindfulness condition would be especially likely
  to form false memories. In two experiments, participants were randomly
  assigned to either a mindfulness induction, in which they were
  instructed to focus attention on their breathing, or a mind-wandering
  induction, in which they were instructed to think about whatever came
  to mind. The overall number of words from the Deese-Roediger-McDermott
  paradigm that were correctly recalled did not differ between
  conditions. However, participants in the mindfulness condition were
  significantly more likely to report critical nonstudied items than
  participants in the control condition. In a third experiment, which
  tested recognition and used a reality-monitoring paradigm,
  participants had reduced reality-monitoring accuracy after completing
  the mindfulness induction. These results demonstrate a potential
  unintended consequence of mindfulness meditation in which memories
  become less reliable.


Official recommendations in the US:
http://nccam.nih.gov/health/meditation/overview.htm#sideeffects


  Side Effects and Risks
  
  Meditation is considered to be safe for healthy people. There have
  been rare reports that meditation could cause or worsen symptoms in
  people who have certain psychiatric problems, but this question has
  not been fully researched. People with physical limitations may not be
  able to participate in certain meditative practices involving physical
  movement. Individuals with existing mental or physical health
  conditions should speak with their health care providers prior to
  starting a meditative practice and make their meditation instructor
  aware of their condition.