The following program demonstrates this problem:

==============< test1.c >==================
#include <stdio.h>

void f(void);
void g(void);

void f(void)
{
      printf("in f()\n");
      g();
      printf("back in f()\n");
}
==============< test2.c >==================
#include <stdio.h>

void f(void);

void real_g(void)
{
printf("in real_g()\n");
}

int main()
{
    real_g();
    f();
}
============== end ==================

$ gcc -Os -c test1.c
$ gcc -O2 -flto -c test2.c
$ gcc -O2 test1.o test2.o -o test -Wl,--defsym,g=real_g
$ ./test
in real_g()
in f()
Segmentation fault (core dumped)

The segfault is caused because there is no real_g symbol and g has been resolved to NULL.

0000000000401146 <f>:
  401146:       50                      push   %rax
  401147:       bf 15 20 40 00          mov    $0x402015,%edi
  40114c:       e8 df fe ff ff          callq  401030 <puts@plt>
  401151:       e8 aa ee bf ff          callq  0 <g>    <---------------- oops!
  401156:       bf 10 20 40 00          mov    $0x402010,%edi
  40115b:       5a                      pop    %rdx
  40115c:       e9 cf fe ff ff          jmpq   401030 <puts@plt>
  401161:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  401168:       00 00 00 
  40116b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)

Seen on Fedora 32 (gcc 10.1.1, binutils 2.34) and Ubuntu 18.04 (gcc 7.5.0, binutils 2.30)