Apologies if this has been fixed already; I got this to occur in either v0.5.8.147 or v0.5.8.74... I would have attempted to recreate the issue in the newest v0.5.8.199 but haven't tried that version yet.
Klaw has this ability:
Forced Interrupt: When Klaw attacks, give him 1 additional boost card for this activation.
Grasping Tendrils reads:
Hero Interrupt (defense): When the villain initiates an attack against you, cancel that attack. If you paid for this card using only resources, stun the villain.
The timing trigger is shared but Klaw's ability is forced so it resolves first and then the player has the opportunity to play Grasping Tendrils. This timing worked correctly but Klaw's ability did not.
When I played the game, Klaw's ability dealt to Klaw an additional boost card, but that dealing of a card should not have happened. It then allowed me to play Grasping Tendrils, and, with the activation canceled by Grasping Tendrils, the boost card stayed on Klaw and was eventually used as an additional boost card for some other activation.
My assertion is that the additional card should not have been dealt.
Rather, I assert that Klaw's ability is a modifier-type alteration effect, that doesn't explicitly or immediately deal a card but instead modifies the number of boost cards Klaw is meant to receive in the upcoming first step of the resolution of the attack activation.
Indeed, giving an "additional boost card" is specifically called out in the rules reference as an example of a modifier.
This rule may apply to more than just Klaw and to more than just Grasping Tendrils.
ALTERATION EFFECT (RRv1.6 p7)
...
This/That Activation/Attack/Thwart — Effects that refer to “this” or “that” activation, attack, or thwart denote a modifier to that activation, attack, or thwart. (Examples: “That attack gains overkill.” “Give the villain an additional boost card for this activation.”)
...
See also: Modifiers
MODIFIERS (RRv1.6 p27)
The game constantly checks and (if necessary) updates the count of any variable quantity that is being modified.
Any time a new modifier is applied or removed, the entire quantity is recalculated from the start, considering the unmodified base value and all active modifiers.
• The calculation of a value treats all modifiers as being applied simultaneously. However, while performing the calculation, all additive and subtractive modifiers are calculated before doubling and/or halving modifiers are calculated.
• If a value is “set” to a specific number, the set modifier overrides all non-set modifiers. If multiple set modifiers are in conflict, the most recently resolved set modifier takes precedence.
• After all active modifiers have been taken into account, if a value is below zero, it is treated as zero: a card cannot have “negative” icons, attributes, traits, cost, or keywords.
• Fractional values are rounded up after all modifiers have been applied.