英语翻译The worst part is,many if not all of these blunders are

摘自《牛津高阶英汉双解词典》（第7版）1014页：
if not---used to suggest that sth may be even larger,more important,etc.than was first stated (表示可能更大或更重要等).
they cost thousands if not millions of pounds to build.建设要耗费数千英镑,甚至可能是数百万英镑.
所以本句的意思是：最坏的部分是,很多甚至可能是所有这些错误都是可以避免的.