Biggest ride cock-up surely has go to to the infamous Vertigo at Walibi Belgium. It opened well over a year late, didn't work, and was taken down and removed not long after.
As for parks, whilst Six Flags' school of management and Hard Rock Park were pretty big cock-ups from a business point of view, for me the biggest cock-ups have been where human life has been put at risk.
The Japanese park maintenance debacle after Fujin-Rajin II at Expoland derailed fatally injuring a rider showed unbelievable contempt for the people paying to go on it, despite having run at serious risk of mechanical failure for many years. Thankfully, it brought about a swift kick up the backside for a lot of amusement parks, which may have resulted in a few places closing down, but with the rides declared unfit for purpose, at least they had no deaths to explain.