So just to clear up the urban legend of the Barista who sucked milk into the boiler, which then had to be entirely replaced at great expense, let's cover exactly what needs to happen to set up the perfect storm.
1-The pressure does need to fall way "below its ideal level". In fact, it needs to fall below the 0 on the pressure gage. This happens one of two way: a) the machine has been turned off, or b) the heating element is no longer functioning.
2-The vacuum breaker valve has to stick shut. The purpose of this device is to allow air back into the boiler in the event that one of the two conditions above have been met.
3-The steaming valve has to be open at the moment, or soon after, the pressure falls below 0. A continued suction inside the boiler could still pull the vacuum breaker open at any moment.
4-The wand must be submerged in a pitcher of milk at this critical time.
It is true that all the planets can align and this can happen. It is true that this has happened. It is true that the boiler then needs to be replaced. However, this is not ever something that happens under normal operating conditions, at not caused completely by slack technique. How would these things ever happen at same time, you may ask yourself?
1-A very old espresso machine, which has never been turned off, and which has been using hard water for many years, has a vacuum breaker that use to hiss a little bit years ago. Calcium and lime deposits, which had been slowly escaping from the breaker, built up a chunk of scale which now prevents the breaker mechanism from opening.
2-Old age has also caused an electrical malfunction, and at 3:27 in the afternoon the heating element stopped receiving eclectic current, so the boiler is cooling off. It is not a busy time of day, so no one has noticed anything unusual. The steam has cooled off so much now that steam in the boiler has contracted to the point that it is no longer pushing out, but sucking in. A vacuum has been created inside the boiler.
3-Someone orders a Latte, and the Barista gets the milk pitcher ready to steam, submerging the tip below the surface of the milk. This is where poor technique has come into play. The Barista should have purged condensed water out of the wand before steaming, and would have noticed that no steam came out. But instead, the moment the steam valve was opened, the barrier between the inside of the boiler (now with negative pressure) and the outside world has suddenly been broken. A rather large amount of milk from the pitcher is drawn very rapidly up the wand, through the valve, and ultimately, into the boiler. It will curdle. It will curdle badly. Even once you fix the electrical and get the thing working again, you can never get rid of the stench of foul milk out of the boiler.
So yes, it can happen. The moral of the story is- Always purge before steaming. No exceptions.
Small amounts of milk will always get in the wand, even with good technique. Milk by itself is easy enough to clean out with regular maintenance. Chocolate milk, or syrup, will coat with inside of the wand with a thicker layer of sticky crud than just plain milk. So I maintain that syrup in the cup is always the best technique for mixing flavors into the drink.
And that my friends, is the end of Barista Bedtime Stories for tonight.
I've actually tried steaming milk & syrup together - I steamed milk with raspberry syrup so I could produce a pink rosetta. This looked pretty cool, but I can't tell you if there was a flavor difference - I took a sip of my drink, found it gross, and never thought to prepare another raspberry latte, syrup-in-cup-style, to compare.
What I will say is that I purged and cleaned the steam wand thoroughly after completing my experiment. It's easy to gunk up the wand when you steam weird things. Also, foreign flavors tend to linger in the wand. Ever make an eggnog latte? Unless you purge and clean the wand SUPER-well afterward, the next 3 lattes you make will taste like eggnog. (Bleh!)
For these reasons (gunking up the wand, unintentionally altering flavor of the next drinks you make) I'd probably stay clear of steaming flavoring & milk together.