In many cases a bad restoration or reconstruction can be far more damaging to an archaeological site than just doing nothing & leaving the site to slowly deteriorate. Not only are items damaged, but often any information linking them to the other artefacts is lost as they are re-assembled incorrectly.
Deliberate vandalism, such as the removal of the sculptures from the Parthenon under the instructions of Lord Elgin is worse – it is like bad restoration, but without good intentions.
Earlier restorations of the Acropolis by Balanos in the 1920s have caused huge amounts of damage to the buildings on the site. Iron clamps were used to join pieces of stone, but without the lead covering that had been used in ancient times. Over time water reached the iron & rusted it causing it to expand breaking the stonework as it did so. The problem was further exacerbated by incomplete records of the cataloguing, that meant that it has taken a long time to actually find all of the replaced clamps.
The current CCAM restoration of the Acropolis has been subject to stringent guidelines & is trying to correct many of the errors made by Balanos.
Burma is not so lucky, as the military government there is trying to rebuild ancient sites, using concrete, brick & bathroom tiles.
BBC News 
Last Updated: Saturday, 4 June, 2005, 01:19 GMT 02:19 UK
Burma rebuilding risks Pagan jewel
By Andrew Harding
BBC News, Burma
The sunsets are still spectacular – a golden glow brushing the curves of 2,000 ancient temples and pagodas clustered on the edge of the Irrawaddy River in central Burma.
But today some of the world’s leading experts have accused Burma’s military regime of waging “archaeological blitzkrieg” against the legendary Buddhist treasures of Pagan.
“They’re ruining it,” said Richard Engelhardt, regional advisor for the UN’s cultural arm, Unesco.
“It makes me feel hopeless and helpless and angry and disappointed,” he said.
I went to survey the damage, posing as a tourist. Burma is one of the world’s most repressive dictatorships and foreign journalists are not welcome.
“We are the richest archaeological site in Asia,” said my guide proudly as we drove around the site in a horse-drawn carriage.
But almost everywhere I saw signs of the “false” and “misguided” restoration work which Unesco and other experts have so bitterly condemned.
- Hundreds of brand new pagodas built with brick and concrete on top of ancient ruins
- A half-built “palace” being constructed from poured concrete at the heart of the site
- The widespread use of bathroom tiles, concrete and other unauthentic materials
- A 200ft (65m) observation tower and hotel complex under construction on the site
“I’m horrified by the tower,” said Mr Engelhardt, who is concerned that the isolated regime’s hunger for tourist dollars is responsible for the changes.
“The archaeology destroyed during excavation for its foundations can never be recovered. The [Burmese] government is gussying up the site… commodifying it for mass tourism.
“But it’s a loss for everyone. It’s becoming less and less a real document of the glory of Pagan’s past and more an un-understandable book of nonsense,” he said.
So what do the locals make of the building work?
Well, remember Burma is a military dictatorship.
“I cannot tell you,” said one souvenir seller with a nervous glance around us, “there are spies everywhere.”
“We all hate the tower,” said another man. “But if we say the government is not very good, we get in trouble.”
Although some locals have found work in the new hotels opening up – built with an eye on luring mass tourism from neighbouring China – many feel they are being pushed out by a regime anxious to monopolise all tourist revenues.
“All the businesses in town are owned by the military,” said one man. “They want to stay on their throne forever.”
For decades Unesco has sought to arrange World Heritage status for Pagan. But disagreements with the Burmese regime have blocked progress and prevented the UN funding programmes to help train local archaeologists to maintain the site.
“The generals have no room for other voices, for constructive criticism,” Mr Engelhardt said.
“There really aren’t the people in [Burma] with the skills to do the job right, to rescue the site. And to me that is the most frightening thing.”