I've got to side with Jhaelen here - the Mummy's effect is triggered in the discard pile, and that's all that counts. The reason why the Mummy got there is irrelevant.
Yithian Scout has his text to limit his effect to be only triggered in the discard pile. It's meant as a limitation, not a clarification regarding Tesla. (There are some effects that may be triggered in discard pile and from somewhere else - see Stalking Hound: "Response: After an opponent's character enters play due to a triggered effect, put Stalking Hound in play from your hand or discard pile.")
Or does the confusion come from a timing problem? Here p. 16 of the FAQ 3.0 is helpful. The timing structure for an Action (e.g. if the Mummy entered the discard pile because of a Shotgun Blast "Action: Choose and wound a character with skill X or lower.") is like this:
1. Action is initiated.
3. Action is executed.
4. Passive abilities
5. Forced Responses
I. Forced Response ability is triggered.
III. Forced Response is executed. (Follow steps I through V, etc.)
IV. Passive abilities are initiated.
V. Other forced response (requirements now met) are initiated.
7. End of Action
The Shotgun blast is an Action and is resolved in step 3 (-> the Mummy goes into the discard pile in step 3). This satisfies the Mummy's Forced Response, wich triggers in step 5.I and wounds all characters in step 5.III. This satisfies Tesla's Forced Response that puts a success token on him in step 5.V.
I hope I could help you with that... timing in CoC is a subtle thing wich takes some time to understand, but afterward frees the player from thinking about the stack as other card games use it (e.g. MtG). FFG's card designers clearly indicate at which time you might play a card (there are some really tough exceptions of course). By this, designers take some mental work off from you. Constructs like Disrupts and Forced Reponses and Responses also exist in games with a stack, but you as the player have to "parse" each card's text correctly and watch out for subtleties like present tense vs. past tense and so on...
Kind Regards - and have fun with the game
Edited by HilariousPete, 18 July 2013 - 11:29 AM.