Strange fscanf bug

[imposterrific]

I'm using fscanf to read data out of a file which I created by writing to the file eith fprintf. Here is an example of my code

----- writing out ------------

char idNum[30];
// here I fill the array
FILE * fout = fopen("data.txt","w");
fprintf(fout,"Id num %s\n",idNum);
fclose(fout);

----- reading in --------------
later in my program I read in the data I wrote out in a loop
testing each id for a match

char idNum[30];
FILE * fin = fopen("data.txt","r");
fscanf(fin,"Id num %s\n",idNum);
if(strcmp(idNum, desiredNum))
...
fclose(fin);

Right now I have three files to which I write the same id number each time. The id number is 97808859948941785921. The first two files I read will read in the Id num correctly, but on the third file and always the third file the id number read in is 0656663696113801664 even though when I open the file and read it myself it says 97808859948941785921. I have other functions which read the id with the same code as above just cut and pasted and it reads correctly.

Is there anything which could interfere with fscanf (and fread, I tried it and got the same result)? I am totally baffled.