Prevention costs
When drilling holes off or reeming holes from outside of aircraft in, you should have someone on the inside of fuel tank with a vacuum cleaner held over the hole to contain the swarf as the hole is being drilled!, for example.
These's days due to time and careless these little details are not incorporated into a lot of todays maintenance practices. In every maintanence task there is many ways of carrying out the job, (unlike the boys flying em) the long way or the short dodgy way. No airline wants to pay for good maintenance. You will see a lot more of these sort of occurances in the future.