A couple of months ago I've discussed some question (the question itself might not be too creative, as a in-some-sense natural although not asked before followup for an open question that was closed last year) with a superstar in my field with whom I'm in decent working relations (and he's a very very nice person).
We've discussed the question and possible directions for a couple of times in emails and a meeting over this two months, had no real progress at all, but in the last one after some discussion he mentioned a direction that we both agreed should be checked. Afterwards I went abroad (academy related) for a few weeks so I didn't get to continue thinking about it.
When back, he informed me that this general direction (after using interesting and non-trivial tools) has worked and that he has solved the problem, started writing it, and will write me an acknowledgment.
Now, as I made no contribution to the right solution or writing, I assume that even if he would offer coauthorship I was supposed to decline. Yet, I'm unhappy for not being informed during the progress and given the chance to make this contribution even if it was clear to him that he can solve it by himself...
As I don't really think "What should I do?" would sprout any answers that will be very helpful, I'd rather ask what do you recommend doing in order to avoid such situations in the future? And also, if indeed I get that supposedly big acknowledgement in a paper written by one of the biggest names in my field - is there anywhere or anyway where it is appropriate or helpful to mention it?