It's not just HSBC, other card companies do the same thing and it doesn't always work even if you let your bank know in advance that you will be abroad . It was a recent subject on BBB'c Watchdog program and I also read an article about this problem in a recent article in The Times newspaper.
Although as to using a card in Germany, I live there and rarely try to use my UK debit card in German shops as I find they are loath to accept it, though I don't have trouble obtaining cash from ATM machines.

Well what can I say it happened to youngest daughter again , last summer in America HSBC blocked her debit and credit card this time after telling them she was going to Corfu and only using her card at the airport ,on the way home she goes to Tesco and guess what HSBC has blocked her credit and Debit cards again .
Sioux

When I was the bank today, the Guy in the window next to me was telling the cashier he was from Austrailia and his card had been blocked. The cashier told him he should have informed his credit card supplier before travelling. It was for Barclaycard/visa.

I am such an earwigger arnt ?

Grrrr , same thing happened to us last week in Florida . Hubby has a Tesco Mastercard and had used it for a week before it was refused after filling car with petrol . Luckily he had another card to pay with but was not amused . Ended up buying a phone card and contacting Tesco . Their reply was that they were only trying to protect us from fraudulant transactions despite my husband telling them TWICE before we went away that he would be using the card in Florida . The person on the other end of the line could only repeat again and again that they were 'only trying to protect us ' . So not a satisfactory answer or result . Hubby was going to phone them up for a better reply now we are home but think he is too depressed about returning to work that he has forgotten all about it , lol.