How are you reproducing this? I'm not getting any issues:

Code:
/write -c test.txt a
/write test.txt b
/write test.txt c

//echo -a $lines(test.txt)
; 3

/fopen a test.txt
/fseek -l a 1
; * fseek set 'a' to line 1
/fseek -l a 2
; * fseek set 'a' to line 2
/fseek -l a 3
; * fseek set 'a' to line 3
/fseek -l a 4
; * fseek failed on 'a'

- argv[0] on EFnet #mIRC
- "Life is a pointer to an integer without a cast"