It was never made into an anime, but a manga that had a dissapointing ending was Ultimate Hentai Kamen (Not porn. In this case, Hentai is used to mean perverted, as it translates to "Ultimate Perverted Masked Man") It's going fantastically up until the last 6 or so chapters, when there's this big time-skip. Then we find out all kinds of crazy shit that just ruins it. Like
(Spoiler Alert. Not sure how to make the thingy so just don't look below)
-Hentai Kamen is not married to the girl they just spent some 100 chapters all but screaming that he and she were in love and going to get married
-Hentai Kamen is married to this ***** no one liked and that they constantly said he does not want to marry.
-Hentai Kamen now has a 2-year old son who shares his powers, so we now have a two-year old running around with panties on his head. That's fantastic.
-Hentai Kamen's now brother-in-law is gone from the series, despite being an actually cool character.
(End of Spoilers)
Not to mention
-There was no point for the time skip at all.
-A helluva lot of issues are left unresolved.
-They never explain why Hentai Kamen did the above things.
-And a lot
Which sucks because it was a really clever, funny series