80s novel. A canine-headed humanoid alien unexpectedly steps out from transportation portal [duplicate]

Question

Paperback novel from late 70s or early 80s.

Cover shows a door shaped portal with an alien (whom I believe had a canine shaped head, but looked and dressed human in appearance.) exiting the portal in place of who they were expecting to exit. [Some error caused this alien to arrive instead of the person they were expecting.]

Humanity has developed instant teleportation system. (Can't remember if it was interstellar, interplanetary, or just on Earth.) It is in regular use, and has been for years.

But there have been a couple incidents. One I remember from the story, a man stepped out of a portal, stated a code word, and when a response word was not quoted back, said something like: "I've been intercepted" or "you're not getting me", and set off a bomb and blew himself up and damaged the local portal room. Much to everyone's confusion, because he was who they were expecting, but no one knew why he asked for a code word, or had a bomb in his briefcase.

The second incident was when the "alien" stepped through instead of the man that was expected. When questioned, the alien stated he was just a routine traveler with no idea of portal operations, but knew that he had landed in a lesser technological reality, because Earth had not figured out how to properly stabilize the portal to prevent these types of errors.

There may have been one other incident. I think the lead scientist investigating the situation may have also been an incident which is what told him there was a problem. Years before, when he stepped through, he soon noticed things were not the same. (Maybe cars drove on the wrong side of the road, or his pet who died was now alive, or his fiancée was now his wife.) But I think he never told anyone about it, until the alien stepped though.

Answer 1

The Infinitive of Go by John Brunner. This was asked about in Book about government-funded research yielding functional teleportation that causes personality changes though that does not mention the dog headed alien on the cover.

This is the cover you remember:

The scene with the bomb is:

“George, it’s great to see you again! I guess there’s no point in asking if you had a good trip, because—”

He broke off. Gunther’s eyes had narrowed with suspicion and his gun was levelled at the ambassador’s navel.

“George!” cried the first secretary. “Is something wrong?”

“Countersign!“Gunther hissed through tight-drawn lips.

“What countersign?” the ambassador demanded. “Nobody warned us you were instructed to require one!”

“Then I have been intercepted!” Gunther cried, and without a heartbeat’s worth of hesitation turned his pistol on himself, while at the same time a thermite charge exploded in his portfolio, destroying the secret data it contained.