Healthcare.gov was a national scandal due to poor performance related to bad requirements gathering.

The $32M Hertz scandal

The $480M bug that killed a trading company in 12 secs

Challenger

Probably a bunch of stories from healthcare

Web software I don't know but there are many "modern software" that were meant to replace COBOL code running on AS400 that ended up never making it to production because they were running so badly and projects ended up being money pits.

Friendster

Wave waves at you from the grave.

Yes, it is hardly ever the web-side of it though.