It can certainly completely deflate what would have otherwise been a comedic delivery. It's actually quite curious how fickle that sort of thing is in that regard (which we see also with general issues of timing in comedy). Around friends who know me (and my stutter) well, there's often an elated amusement after the failed attempt though, since we can appreciate the irony of failing right at the crucial moment. In other situations, there'll often be an awkward period of readjustment afterwards, but it's not like that doesn't come with the territory anyway. It's just maybe more visible when the communication falters right at such a key moment.