Ah yes, I remember. Torrent clients do this as well. Though they can only check the integrity of the file, not whether it's an identical copy of the original file, as this would necessitate another download.

You are aware of Checksum functions, right? The method you check if a file you got is the same as the original, without needing to have both at the same time. P2P clients use one, both for finding files and for verifying, though some functions are worse than others (see UUHash).

I'm sorry for my tone.
Which, upon rereading, has been hostile and entitled undermining what I'm trying to do (provide criticism that is constructive as opposed to just being a dick).

I should probably have caveated my post with "I'm on vast quantities of painkillers for a collapsed lung".

No offence was intended.
I am a big fan of GOG. I like having a place where I can legitimately buy old games. I like GOG. I like the service.
I did not mean to seem entitled and crabby.

I had part of a lung removed once. I was supposed to get an epidural for the pain and they placed it wrong. Took them 3 days to realize it. XD Point being, my heart goes out to you and apology accepted. We will do everything we can to help you with your downloads. Well, they will. I'm a tech idiot. :P

Thank you for saying that, and welcome to the forums.

I'm not going to be of any help here, apart from chiming in to say that I have often used the downloader and have never had any trouble with it, which makes me suspect either your connection or your machine, or your relationship in general with the Internet gods. You did remember to sacrifice the goat, right? Some people forget that part. It's a pain, but you only have to do it once.

I'm also a little confused by the mention of pop-ups, which I have never encountered apart from the New Message box, which is a fresh window. I actually like it that way, because it allows me, while writing, to look for things on the forum I might want to include in my post. Never really thought of it as a pop-up, though I guess it is one.

I've been using computers since the VAX my dad ran in his study as a demo model in the 1980s. I was learning the basics of programming from the age of five or six on said VAX.
Connections that aren't great (like a cellphone on 3g) can cause it.

It does not happen very very rarely.
It happens frequently enough that we have functions built into pretty much every programming language ever to check that the file (or data) was transmitted correctly. Checksums, parity archives were really popular in the Dial up days.

Oh well, I must have been really lucky then not to have encountered any errors. :)