I mentioned the Orioles collapses in another post. From the Mets perspective, they have had their share of collapses.
1972 - Started 30-11, then went 53-62, finished 83-73 (Strike shortened regular season).
1980 - Started 56-57, then went 11-38, finished 67-95.
1991 - Started 53-38, then went 24-46, finished 77-84.
2007 - Started 35-19, then went 3-13 to go to 38-32. They were 83-62, then went 5-12, finished 88-74,
2008 - Started 86-67, then went 3-6, finished 89-73.
On the other side:
1985 - Started 38-35, went 60-29, finished 98-64.
1973 - Started 53-66, went 29-13, finished 82-79 (lost World Series)
1969 - Started 9-14, went 91-48, finished 100-62 (won World Series)