Watch for hydraulic cylinders that are leaking as they can leak into the cavity. Also beware that over-lubricating mold components can result in contamination of the cavity with the lubricant. Many times when a mold comes back from being serviced the tool shop will have taken the approach of “if a little grease is good a lot is better”, and this can lead to hours of defective product being run due to the contamination of the lubricant.