Why does as.POSIXlt return NA for some valid dates?

Question

I can't figure out why I'm getting TRUE from this is.na test:

is.na(as.POSIXlt(x = "03/09/2014 2:00", format = "%m/%d/%Y %H:%M"))

When this one gives me FALSE:

is.na(as.POSIXlt(x = "03/09/2013 2:00", format = "%m/%d/%Y %H:%M"))

What is it about March 9th, 2014 that R hates so bad?