What has happened
You've got manure in the milk. That batch is no good, so you'll have to dump it. Then you need to clean everything the manure touched.
Coliform grades are often traced back to exactly this moment, when a cluster drops. It's more likely early in the season, when cows are messier, the pit floor is wetter and everyone is still warming up.
How to confirm it
Usually you already know. A cluster has landed in muck, or you can see manure in the milk. If a cluster hits the deck, assume it is contaminated.
What to do now
Take any cluster that landed in muck out of action until it has been cleaned and recirculated properly. A quick rinse and a hope for the best isn't enough.
Scrub the clusters that fell into manure, then the jetters and any other fittings it reached. Use a separate brush for the milk line, because dirty brushes are bacteria bombs.
Clean and sanitise all the clusters, lines and surfaces the manure affected, using alkali and a vat cleaner.
Bomb wash the plant straight away, with a high dose of alkali followed by a high dose of acid. You may need a vat cleaner if it foams up.
Run the alkali first and then the acid, never both at once. Wear gloves and safety glasses when you handle them.
Check your milk company's grade notification at the next collection to confirm you're clear.
How to stop it coming back
Treat every dropped cluster as contaminated. Flush it with hot water and sanitiser before it goes back on.
Take extra care early in the season, when the pit is wetter and the cows are messier.
If you wash cows, do it early before the cups go on, wash only the dirty ones, use clean water and dry the teats. Don't hose straight into the pit. The aim is to stop mud, muck and bacteria moving from the teat into the milk.
Get the full steps
Your cups hit the deck. The Dairy Handbook chapter How do you deep clean after manure contamination? has the full deep clean, step by step, with the dose rates. How do you bomb wash a milking plant? covers the follow-up wash. Which milking habits cause milk quality problems? covers dropped clusters and cow prep.