When I see a spider I'm screaming.
When I saw a spider I was screaming.
The so-called "simultaneity" readings of the subordinating conjunction
when are always pragmatically modulated by our world knowledge (our pragmatics). Often, a contiguous or nonoverlapping interpretation is primary and comes naturally when due to some logical connection between the situations described in the clauses, e.g. a causality relation. Causes and effects, by their nature, cannot overlap.
I should also add that in this example the narrative reading of
when is also possible even though some contextual support could be helpful in the pursuit. Just assume that the poor thing was screaming for some other (unknown to the reader) reason, and, all of a sudden, the poor thing spotted a spider. Well, double whammies happen. Just realize that in this case “when” abandons its subordinating conjunction role and becomes a link, more coordinating than subordinating…