This only ends up being a problem if you spend a really long time in grad school, but it is something everyone should be aware of. It's tucked away in the fine print, so go hunting for it... It's always a good idea to be fully informed.
Granting agencies impose expiry dates on your eligibility, depending on how long you've been in the post-secondary system. For example, because I spent so many years (many longer than I ever wanted to) in grad school, in my last year of the Ph.D. program I was no longer eligible to apply for NSERC post-doc funding, even if I had been 'excellent' by their misguided standards.