I've been noticing that on chromium, if I hide a thread and it happens to be the first thread on the first page, it'll instead hide the first thread of that page instead.
For example lets say I go to /g/ and see that the first thread on the first page is some troll posting guro. I hide it and refresh the page, and whether that thread is still the first or not, the first thread will be hidden.
I'm assuming it's a problem with chromium, but thought I'd let you know, in case you didn't.