He picked up the glasses because Bain complained of not being able to see and asked for his glasses, Detective Terry van Turnhout told a jury in the High Court at Christchurch yesterday.
As soon as he picked them up, he realised his mistake, that the room was a crime scene, so he immediately put the glasses back on the chair, Det van Turnhout said.
But when a senior officer came into the bedroom and asked if anything had been touched, he told him nothing had been, apart from the pillows.
Under cross-examination by defence counsel Paul Morten, the officer said in the 15 years since the shootings at the Every St house, he had not mentioned until last weekend that he had picked up then replaced the glasses.
"I was worried I'd picked them up.
"I didn't know what their significance was," Det van Turnhout said.
And he said he had no idea the glasses would later become significant.
"I was concerned about being criticised later.
"I made an error of judgement," he said.
Then a uniformed officer, Det van Turnhout was assigned to stay with David Bain and note anything he said or did while in his bedroom and after he was removed from the house on a carry chair about 10.30am and taken to the police station.
The witness said he arrived at the house at 8.03am and took over from another police officer, who was in the bedroom with Bain and two ambulance officers.
When he first saw Bain lying on the floor at the foot of a double bed, covered with several blankets including a thermal type aluminium survival blanket, he initially thought Bain was in shock as he was very pale and saying nothing.
But his breathing appeared normal and his eyes were sometimes open, sometimes closed.
At 8.15am, Bain said he had to get up and go to university, that he studied music and sang.
Bain may have sat up at that point - "I thought he was in a daze", Det van Turnhout said.
The lighting in the room was poor but he could see a rifle cover with its zip half-open on the wall.
He also saw an ammunition belt containing .22 calibre rounds, a number of loose rounds on the floor and a trigger lock with a key in one half.
The bedroom was "cluttered" with tramping packs, camping equipment, books, and other items scattered about.
Looking out through the bedroom door, he could see directly into a lounge room across the hallway.
There was a man's body on floor and a .22 calibre rifle with silencer on the floor.
Some time later, Bain complained about a pain in his head that felt like a bruise and the witness said he was able to see a bruise, about the size of a 50c piece on the right side of his forehead.
Asked if he had noticed anything about Bain's clothing - a white T-shirt, black shorts and white sports socks - Det van Turnhout said he had not, but after one of the ambulance officers drew something to his attention, he saw a small spot of blood on one of the socks.
Shortly after 9.12am, he heard Bain saying "black hands are coming to get me".
He said nothing else but he appeared distressed and disturbed.
An ambulance officer brought hot packs and several blankets for Bain and soon afterwards, he wanted to get up but, when it was suggested he lie on his back, he did that and was quiet.
Bain's eyes were open and he was looking about, then he asked for his glasses, saying "I can't see", the witness said.
Having seen a pair of glasses on a chair, Det van Turnhout picked them up before immediately replacing them when he realised the room was a crime scene.
He told Bain they would get his glasses later, the witness said.
He then noticed there were no lenses in the frames and there was one lens on the seat of the chair.
Bain was removed from the house at 10.20am and taken to the police station where he was interviewed by Detective Sergeant Greg Dunne.
At one point, the accused asked for a pair of sunglasses, saying the light was hurting him.
And he said he had a headache, but not a migraine, and pointed to the right side of his forehead.
He also said to Det Sgt Dunne,
"My father's dead isn't he. I saw him, I saw him".
Some time later he asked if he was going to be held up for long at the police station, as he was in a play and had been rehearsing.
During the time he was with him at the house, Bain had not mentioned his family, Det van Turnhout said, but he agreed he had not raised the matter as it was not his job to do so and he did not want to distress the young man.
