For various reasons, it never bothered me as much as it did other fans.
Firstly, it's our most hated rivals indulging in a bit of piss-taking. Are we saying that rival fans aren't allowed to take the piss out of each other or is it only acceptable when it suits?
Secondly, it highlighted their obsession with us, something that they're so often quick to deny. While those cunts were winning everything and we were dogshit, we still got under their skin enough for them to make banners about us and sing songs about us.
Thirdly, as you say these things do tend to come along and bite you on the arse and eventually it did. United fans should know this more than most after Liverpool fans displayed a banner in 1994 following United's first league title win for 26 years the season before. That put United on 8 league titles and the scousers had a banner saying "Come back when you've won 18", no doubt thinking that it would be 40,50, or even 60 years before that happened when they actually managed it in about 15.
One thing I would add is that Ferguson and United fans in general had absolutely no fucking right to complain about our club doing the the Tevez poster when their club was absolutely complicit in allowing that banner to remain in the stadium for years, and often indulged in piss takes around our trophy drought on United's official website.