A call to BeginUpdate must be accompanied by a subsequent call to EndUpdate, or desynchronization can occur. One way of making sure that both calls stay synchronized is to use a finally block:

try 
{
  i.BeginUpdate();
  // Do some modifications to the zip file
}
finally 
{
  i.EndUpdate();
}

In C#, the preferred way is to use the Xceed.FileSystem.AutoBatchUpdate class within a using statement.

Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

ZipArchive Class
ZipArchive Members
Overload List