Firstly I'd like to start by saying I'm not vegetarian so please excuse any ignorance I might be showing here.

I'm moving in with two friends on Friday and one of the lads is a vegetarian, and his long-term reasonable-distance-away girlfriend is a vegan so will occasionally be staying with us.

I've been out to buy some wine so we can toast to the new house, and upon texting the vegetarian housemate to tell him this he asked if it was vegetarian.

At first I thought he was pulling my leg (wine being grapes of course) but then I looked it up and it turns out that in a lot of European wine they use animal products in processing it.

Luckily I've found some vegetarian/vegan wine so that particular crisis is over, and more of the other wine for me

Are there any other seeming harmless products I should be aware of before putting my foot in it again??

Wine isn't the only alcohol but beer as well. I don't drink so it isn't really something I look out for. Cheese is another item that isn't always vegetarian.

There are powders and so on that clarify wine that come from the inner guts of slaughtered baby cows and so on. These are also used in the production of some sugar, so a lot of veg*ns also have to be careful of what brand of table sugar they buy. This is the reason, when I describe myself, it is as a whole food plant based diet. I actually am a Vegan, but I am too lazy to look too deeply into hidden ingredients. To me, beer is a grain drink, and wine is a grape drink, and I do not look too deeply into the processing. I do not eat the food if the animal derived food is a major ingredient like gelatin in yogurt.

Yes, there are a bunch of food additives (referred to as "E numbers" in Europe) with strange names which are animal-derived. As giselley wrote above, gelatin -- made from the skin and hoofs of animals -- is often found in sweets to give them a chewy consistency. Other stuff to watch out for include carmine/cochineal (made from insects to give foods a red color), casein/sodium caseinate (made from milk, used as a protein supplement), and L-cysteine (made from hair and feathers, used in bread). You can find a full list of non-vegan food additives by googling it. It's not as arduous of a process as it may seem.

There's a website called barnivore.com which has made it a lot easier to find vegan alcoholic drinks. You may search for the brand of wine, beer or liquor in question and find statements made by said brands on whether their product is vegan. If it's vegan, you'll see a green box next to the brand name; if it's not, the box will be red.