It's inevitable that in fifteen books there will be a few mistakes. Some such 'mistakes' are explained in the FAQ. Others are exactly what they appear to be: mistakes.Here are a few I've noticed. If you can explain one of them, or know of another, let me know.
In The Dolphins of Pern , towards the end, Oterel, Lord Holder of Tillek, makes an appearance on the boat to offer Readis his support of a Dolphineer Hall on behalf of the Lord Holders. This would be fine if Oterel hadn't died in All The Weyrs Of Pern and been succeeded by his son Ranrel.
In Dragonflight, Lytol's lost dragon Larth is referred to as a green. Later he magically turns into a brown.
In The Chronicles of Pern: First Fall, it is mentioned that Tarvi Andiyar, or Telgar, had formed a mining settlement. It wasn't yet a hold but it would have become one soon after. In previous books Telgar is referred to as one of the newest Holds, but here it is apparently one of the oldest.
First Fall also reveals that South Boll was formed before Ruatha, but in the earlier books Ruatha is referred to as the second oldest hold.
In Dragonflight the actual main hold in Ruatha is referred to as 'Ruath Hold'. The province, or whatever, is called Ruatha, but not the main hold. In subsequent books it changes to Ruatha Hold. It's impossible that the name changed between Dragonflight and later books, because in The Ford of Red Hanrahan Red names the hold Ruatha.
Again in Dragonflight, Lessa tells F'lar that Kylara named her son by T'bor, T'kil. But shortened names are an honour given only to dragonriders, not all weyrfolk. The baby would have to be given a full name like Torkil, which would then be abbreviated if he Impressed.
This isn't really a mistake, more a quibble. But on the front cover of the Corgi edition of Dragonflight, Mnementh is pictured as being slightly bigger than Ramoth. However, queens are meant to be much bigger than bronzes, and Ramoth is the biggest dragon on Pern.
In the foreword in Dragonsong, it is implied that dragonmen lived in the holds until overpopulation forced the Pernese to expand eastward. The discovery of what would become Benden Weyr and how it suited dragonmen's requirements prompted them to found other Weyrs in similar extinct volcanoes. We know from Dragonsdawn that Fort is the oldest Weyr, though.
That's all I've got for the time being. I know there are more, so I'll put them up when I find them or someone tells me about them.