According to Wikipedia:
People with arachnophobia tend to feel uneasy in any area they believe could harbor spiders or that has visible signs of their presence, such as webs. If arachnophobics see a spider, they may not enter the general vicinity until they have overcome the panic attack that is often associated with their phobia. In some extreme cases, even a picture or a realistic drawing of a spider can also trigger fear. Arachnophobics may also be touched by or touch an object and be afraid because it could feel like a spider. Arachnophobics may feel humiliated if such episodes occur in a social setting.
That may may sound a bit extreme, but maybe it wouldn't be so unreasonable if you were walking along at night and your face went right into the center of a huge spider web.
Sure, you could google it afterwards only to find out it's a harmless garden spider, but the trauma genie would already have wreaked considerable havoc outside the bottle.
If it wasn't for my yard light reflecting off the web, I just might have caught a case of arachnophobia ...