I’ve fallen foul of this a few times. 
Just to clarify, it’s the first post of a topic that suffers from this limitation (as the ignored replies get tucked up into ‘hidden replies’ and can be expanded if you’re ever curious enough).
If a topic is public you can have a look in a private browser window, which is do-able but still a faff. Though also no help if the topic relies on being logged in to view. Other than that, you have to do the ‘un-ignore → ignore’ shuffle.
A similar mechanism to the flagged posts would be a nice addition for these:
(I’m sure this has cropped up before but Related Topics is not helping me out, and my Search Fu skills are not what they once were 
)
Edit: I did find this, though not the one I think I was thinking of… 
But helpful to link up and boost the rule-o’three: