Unclear handwritten ID numbers can be a problem to read/interpret. So far I’ve seen and puzzled over:
How about you? 
I often get this one 
Once the numbers were correct, but the country code was wrong (but so close that I didn’t notice it at first - partly because I was a newbie and did not know them by heart), another time a couple of digits had switched places (tried different combinations a bit, but not the correct one) and twice there was no code. In all of these cases I was able to deduct who the card was from and the senders gave me the correct code. (Yes, I know one can ask for help from the team and if I was clueless, I would, but it was fun playing a detective!) Sometimes I am not sure if a number is 1 or 7 or if another is 6 or 8, but one can always try them out. It was not in postcrossing, but once I had to intepret a squiggle that looked liked a stretched S or a lighting with bends instead of sharp edges. It could have been 2 or 4, but when I asked the writer, it was number 1…
Oops, I must confess that I write like this, haha!
Those are often from the Netherlands. I was confused about them as well in the beginning.
Did you know that the 1 is commonly written like that in Germany? Well not a real triangle, but that might happen when writing faster and / or adapting to a personal writing style.
This is how we learn it in primary school 
Before I started Postcrossing I wasn’t aware of the fact that numbers are written differently in different countries. But there are a lot of varieties around. Same as for letters (especially cursive).
Yeah, I was going to comment that it was probably from a German!
It’s funny, somewhere along the line I adopted the practice of putting a line through my sevens even though I don’t write my ones the German way.
I have had that happen as well, and it takes some extermination to get it right. That is why I always cut out the ID number and glue it on the card and love it when others do the same.
I love the line, because a 7 can’t be mistaken for a 1 this way 
But yes… that’s typical for German numbers, too! (but we don’t learn it at school - there might be exceptions - , so I don’t know where it comes from!)
That is how I learnt it at school in Italy too. Nowadays I alternate between this and the simple straight line, and I am conscious of doing single straight line only when I write to Americans. I always and only do a 7 with the line in the middle, again that’s how I learned to write and I cannot bring myself to do otherwise (not that I should, but just to say that it is not variable like my 1s).
I thought it was all of Europe (except the UK) that does it that way (1 not just a straight line and and 7 with the bar), so not just Germany. If I am incorrect on that being so frequent, then at least you have Germany and Italy 
This is also how we are taught to write these numbers in Poland.
I find it very interesting to see how the numbers looks just slightly different in each country! There has been even time I could see the sender’s country just by looking at the numbers (well, of course the country ID is right there too, but you know)
I am afraid I might be the one sending unclear ID numbers. I have heard many times that my 4 looks a lot like 9. I try to be more careful when writing addresses and ID numbers.
I do my 1 as just a straight single line but 7 has a bar. The straight line 1 might be just me, I think we might have been supposed to do it with a hat at school, but I was too busy for that.
But what I find annoying is when people write the ID next to the address or even have it printed on the same piece of paper. The mailman seems to be annoyed by that too as sometimes the ID number is crossed over if it has been placed right next to the address. That makes the ID very unclear.
YES. OMG why do people do that? I am so paranoid about the address being unclear that I make sure nothing stands in its way, how do people think that it is a safe place for the ID number? Does it not confuse the sorting machine/mailman/whoever?
And it happened to me MANY times that I was about to ask for help from Postcrossing to locate the ID, when in the end it was just above or near the address (it’s not like I go and read my address carefully…) - once I even had the contact form open and just while filling it in I looked closely enough and found it 
That’s a 4 ? !
Yes 
My phone number has 4 too… That is why I know…
The one I had with a legible number but no country-ID was from a complete newbie. I worked out the ID from the postmark and registered the card, in my thank you message I said she should put the ID for Indonesia (or whatever it was). She replied immediately saying she hadn’t realised that. 
Yes it was, here is part of it. Do they look like 8s? I seem to remember in the ‘old days’ when computer zeros had lines through them to distinguish them from capital Os.
Those are extreme. I think I have yet not encountered Dutch 8s as unreadable as that.
Not a problem with unclear IDs but some postal worker seems to have a problem with how my street number was written. I often get my cards like that:
Ahah they cannot cope with the non-European way!
The crossed out on would also be like they do in the UK. I would do it like the one written on the right except that I mix my 1s as I said.
I cannot quote the picture for some reason, but I am shocked about those Dutch 8s, I have never seen anything like that in my life! 
Here are some 1s from France: (triangle with no base)
and some ? 4s ? from Russia.
