[elit]

If you ever compressed with what came as a default setting in arc.ini for srep(and may be also relevant to other tools as well):

[External compressor:srep64]
packcmd = srep.exe {options} $$arcdatafile$$.tmp - <stdout>
unpackcmd = srep.exe -d {options} - - <stdin> <stdout>

Keyword is that {options} string inside unpackcmd(not packcmd).
Then be aware that if you remove that {options} from the unpackcmd, you will NOT be able to decompress IF you used some fancy options like :l or :c during compression(and curiously with others like :mem or :a or :m it will still work)!
You would be thinking that it doesn't make sense to have an options substitute in unpackcmd(and you are right, not sure why that was in default arc.ini) and since you wouldn't be able to use it anyway you remove it later on.
You would be also thinking that since you did NOT put any option during unpacking(because you can't anyway) then it cannot be different and should be irrelevant whether that string is there or not.... well NO. Because if it was there during packing, you have to have it during unpacking.
If you packed without the string then there is no problem, it will work without it(and interestingly, it will unpack archives fine that were compressed without the string during compression even if the option IS there during decompression).

In other words:
- during compression: unpackcmd containing {options} == have to have it there during decompression.
- during compression: unpackcmd NOT containing {options} == does not matter it will unpack OK regardless.

Above means that packcmd and unpackcmd are not completely independent. It also spell(to me) the string substitute issues, even with empty string and a shoddy coding practices. So be aware of it, if you can't decompress something in the future.