An elderly patient died after drinking Flash cleaning fluid from a jug on her bedside table, an inquest has heard.
Joan Blaber, 85, died on 23 September last year while being treated at the Royal Sussex County Hospital in Brighton.
Coroner Veronica Hamilton-Deeley told the inquest it was no exaggeration to say "no-one can actually say what happened".
"This is incredibly worrying," she said. "It could happen again."
The hearing at the Jurys Inn hotel heard Mrs Blaber, who lived in Lewes, had been admitted to hospital on 22 August with a suspected minor stroke.
She ingested the liquid on 17 September and died six days later.
A post-mortem examination concluded she suffered respiratory failure.
The widow, a retired shopkeeper, had been housebound since the beginning of the year with arthritis and chronic leg ulcers, and she had been treated for cancer.
The coroner described the inquest as "hugely complex" and "difficult".
"Even the most rigorous police investigation has not completely ascertained what happened," she added.
In May, Sussex Police said its investigation had been completed, there was no evidence of a crime being committed and there were no further lines of inquiry.
But at a pre-inquest review in June, Ms Hamilton-Deeley said a "deliberate act" could not be ruled out.
Brighton and Sussex University Hospitals NHS Trust immediately removed Flash from the Royal Sussex after Mrs Blaber died.
In May, a report by the Care Quality Commission criticised the trust for not always keeping cleaning chemicals safely.
In response, the trust said it had taken "significant steps" to improve the way it managed potentially hazardous substances.
The inquest continues and could last up to eight days.