I didn't have the dream personally, but I remember my friend's mother, who helped me tremendously in the making of my Relius Clover costume, had a nightmare where the staff at the convention, the one we were making the costume for, wouldn't let us in unless we were 23 years of age (i'm 21, so even then i'm like huh?).
so yeah. try to avoid doing cosplays the week before a convention. they give nightmares, yo.