I hate to say it but I actually do not like that game mechanic, it was funny the first time, but just the first time.
I know you can react to it, but the thing is you're never prepared for the spider and if you're going for flawless run, you will lose it just because of that spider.
Only in stage 1. A flawless run in any other stage is not 0 deaths.
Klazen sent me the achievement list i'm completely aware of it, however it still can fuck you over in a flawless run 0 deaths or not, so you can't use stage 1 as an excuse. If i were to list examples where the spider would catch you completely unprepared, you have 5 deaths in stage 2 and are at the last boss of the stage, you get a spider and are unprepared, gg.
I'm sorry there is just, nothing good to say about this spider, i know it is a low chance of spawning but it still can happen and usually you're never prepared. Though not the "worst" game mechanic, though sadly enough, it is far down there in my list of opinions. I don't think I've ever seen anyone react to the spider as a matter of fact, i'm sure people have done it, but I've never seen it done.
Though, stage 1 flawless is the only thing you had to say about it? I hate to say it, but that's a very weak defense on the mechanic, you can easily get around it and still argue it for later stages because like i said, it adds 1 to that death counter and will make things harder for you and make you more cautious.