I cannot remember reading about movies that cast actors so late in the production!
I don't think it's that uncommon. After all, it can take months to shoot a film, so you don't necessarily need to cast a role before shooting if you won't be filming their scenes until later in the process.
There have, of course, been cases when a role was recast shortly after shooting began, like Hugh Jackman replacing Dougray Scott as Wolverine and Viggo Mortenson replacing Stuart Townsend as Aragorn.
Good points. Michael J. Fox was signed on to Back to the Future after they had already began filming with Eric Stoltz.
Anyway, I am happy with this cast. I think they made some great choices.