In one hospital, staff were alarmed to learn that a volunteer had been proselytising to a patient.
Elsewhere, things turned sour when another was overheard informally advising a patient on complementary medicine.
Such incidents, though rare, are risks undertaken by hospitals when they accept volunteers.
While those actions may have been done with good intentions, they fall out of step with public hospitals' rules on volunteers' conduct.
They cannot, for instance, impose religious views or sell insurance to patients, their families and staff, said Ms Mumtaj Ibrahim, senior manager of community relations at Singapore General Hospital (SGH).
And volunteers who put up a play or concert cannot perform religious scenes and songs.
Neither can they take photos with patients, said a Changi General Hospital spokesman.
Volunteers also cannot discuss patients' backgrounds and medical conditions in public or on social
media platforms, she added.
Such rules are usually relayed during briefings, orientation or training sessions held for volunteers before they begin their service.
At Yishun Community Hospital, volunteers undergo a three-hour orientation and training that cover the dos and don'ts, plus topics like using wheelchairs, said its volunteer coordinator, Ms Linda Ong.
For the incident in which the volunteer was found to be preaching to a patient, the person was counselled and has since left the hospital, said Ms Mumtaj.
She noted that most volunteers at SGH comply with guidelines.
But there are times when some people need to be reminded or counselled on the appropriate behaviour and attire, for instance.
"Should the same issue persist, we may provide re-training or assign the volunteer a new role that suits him better," said Ms Mumtaj.
"We will relinquish the services of the volunteer if our repeated attempts to work with him fail."
Besides proper conduct, hygiene is also of paramount importance.
Volunteer Yong Yung Peng, 46, who helps out at the National University Hospital with her husband, said she initially had "irrational worries" about bringing diseases home.
But her training covered this aspect, added the senior human resources manager. The session also taught how to handle and interact with patients with care, and appropriate conversation topics.
Said the mother of two: "However, we learnt most of what we do through observation, playing by ear and conversing actively with the nurses, patients and their family members."
Poon Chian Hui