Never having thought about it, I just found out that I am used to Shift-Space doing the same thing as Space. But now it doesn't any more: my typing started looking like this a couple of days ago:
a[x +1]= b
where I wanted to write
a[x + 1] = b
I suppose I let go of Shift too slowly. It never used to be a problem, until I installed Yosemite, and now Shift-Space does not register at all any more. I tried :imap <S-Space> <Space>, but that too does not trigger.
It does not happen in Terminal Vim, nor does it happen in any other application I own.
EDIT: I am on a Japanese keyboard, using Kotoeri (Apple's Japanese IME).
This seems to be an incompatibility introduced in Yosemite's Kotoeri (Japanese IME). For the time being, switching to Google Japanese IME as an alternative (or not using Japanese input in the first place, if that is an option) circumvents the issue. See this discussion for details.