hm, i dont think that's ever happened to me.
i mean, occasionally if a bit taken aback, i might not be able to speak. like if someone says hi to me, it may not come out when i say it back. i say it in my head. and then by the time it occurs to me to get it out, it's too late.
actually, maybe that HAS happened in a small dose. it's like, the words just cant come out, right? this is when im nervous, on the spot, or unprepared to answer. i think all humans experience this in small quantities, though, dont they?