Going back on the topic of CtM/slums, I’m just going to drag this execution flow over from the main discussion thread (courtesy Jako), outlining how the effect works:
1. The runner pays the trash cost of the card to trash it 1.1. Salsette Slums meets its trigger condition and triggers 1.1.1. Salsette Slums resolves and puts its replacement effect into play 2. The card is trashed 2.1. Both Salsette Slums and Controlling the Message care about this game event. CtM meets its trigger condition, but has not yet triggered because there is a simultaneous effect to resolve first. 2.1.1. The constant effect from Salsette Slums is faster than triggered abilities. It replaces the card being trashed with removing that card from the game. 2.1.2. CtM goes to trigger and can't because its trigger condition is no longer met.
… Sometime later that turn …
42. The runner pays the trash cost of a second card to trash it 43. The card is trashed