I've been in a company when they asked us to work weekends. It didn't work out: the real reason for the delay was political (CTO dropped the entire backend because it wasn't in the language he liked) and they kept being late until the investors pulled the plug.

In other companies I've been at the cause was that the codebase was atrocious. Not much to do there if not to keep working as usual. Business kept pulling out resources though.