Some patients 'wake up' during surgery
More than 150 people a year in the UK and Ireland report they have been conscious during surgery - despite being given general anaesthesia.
In the largest study of its kind, scientists suggests this happens in one in every 19,000 operations.
They found episodes were more likely when women were given general anaesthesia for Caesarean sections or patients were given certain drugs.
Experts say though rare, much more needs to be done to prevent such cases.
'Unable to move'
Led by the Royal College of Anaesthetists and Association of Anaesthetists of Great Britain and Ireland, researchers studied three million operations over a period of one year.
More than 300 people reported they had experienced some level of awareness during surgery - some recalled experiences from years ago.
Most episodes were short-lived and occurred before surgery started or after operations were completed. But some 41% of cases resulted in long-term psychological harm.
Patients described a variety of experiences - from panic and pain to choking - though not all episodes caused concern.
The most alarming were feelings of paralysis and being unable to communicate, the researchers say.
One patient, who wishes to remain anonymous, described her experiences of routine orthodontic surgery at the age of 12.
She said: "I could hear voices around me and I realised with horror that I had woken up in the middle of the operation but couldn't move a muscle.
"While they fiddled, I frantically tried to decide whether I was about to die."
'Rare but concerning'
She told researchers that for 15 years after her operation she had had nightmares of monsters leaping out to paralyse her.
And it was only after she made the connection between this and her operation that the nightmares stopped.
Each person's experience was analysed to identify factors that could make these situations more likely.
About 90% occurred when muscle-relaxant drugs - used to help paralyse muscles during surgery - were administered in combination with other drugs that normally dampen consciousness.
Researchers believe in some of these cases patients received an inappropriate balance of medication, leaving them paralysed but still aware.
And there were several reports of awareness from women who had Caesarean sections while under general anaesthesia.
Though this type of anaesthesia is most often used in emergency situations, researchers say women should be informed of the risks.
They calculate up to one in 670 people who have Caesarean sections with general anaesthesia could experience some levels of awareness.
But experts argue this is partly due to the balance needed when achieving unconsciousness for the woman while still keeping the baby awake.
Other common factors include lung and heart operations and surgery on patients who are obese.
And some 17 cases were due to drug errors.
Researchers are calling for a checklist to be used at the start of operations and a nationwide approach to managing patients who have these experiences.
Prof Tim Cook, at the Royal United Hospital in Bath, who led the research, said: "For the vast majority it should be reassuring that patients report awareness so infrequently.
"However for a small number of patients this can be a highly distressing experience.
"I hope this report will ensure anaesthetists pay even greater attention to preventing episodes of awareness."