John Ioannidis published a now classic article Why most published research findings are false.
An easily digestible write up of that paper appeared in the journal Atlantic. Lies, damned lies, and medical science.
Both the above are must reads for anyone embarking on a career in medicine or bioscience.
Now, John Ioannidis followed up his previous work with a new article published this week: How to make more published research true.
I highly recommend reading all of these.
One reason why published research is wrong – cognitive bias:
(For evolutionary medicine students, the links above are optional extras for discussion only)