I had a 6 month placement in a food testing lab just after getting my undergraduate degree. We tested food and drink samples from a variety of companies to check for harmful bacteria. The tests involved growing bacteria from the samples and conducting various tests (dependent on the sample type and the likely contaminants). As the tests generally would take at least a day or two to perform, products could not be held up while they were conducted, so the tests were essentially retrospective - we would be checking that products which had already shipped were ok.
One of our customers was an ice-cream company. One of the tests we performed was to to measure thetotal bacterial counts within set product volumes. The counts were always within acceptable levels, but we noticed that over time these counts were increasing. When we pointed this out the the company, they immediately checked their factory and found the cause - a lump of ‘crud’ had found its way into one of their machines. Because of our monitoring they were able to catch this before any serious issue developed. Left unchecked, the bacterial contamination would only have increased over time to the point where a health risk may have developed.