Originally posted here by HTRegz
So you'd start with:

"Mirc version 6"
goto 12
blah blah
12: more code here

Size = 45 bytes (who cares what the numbe ris

and you'd end up with

"Mirc version 6 OMFG"
goto 12
blah blah
12: more code here

Size = 50 bytes
This would mean you've changed the source of mIRC and recompiled. If you used a hex editor you will be overwriting other (maybe important) parts of the binary. Even if you just inserted the bytes some of the (relative) jumps or lookups inside the code will all get screwed up (jump half way inside the wrong subroutine, lookup the wrong value i.e.).

A bit of pseudocode:
Code:
.data
text1 db "This is a text\n"
value1 db 12

.code

 load Reg1 value1
 print text1
 end
If you inserted text inside text1 the address of value1 should also change. If you did this to a binary Reg1 wouldn't contain 12 anymore (but part of text1) because the address of value1 is calculated at compile-time not at run-time. That would explain the crashes.

Hope that explains it a bit