The key impetus for the problem that confronts the Bureau in TSS is that Shapiro had been part of the rather productive Leningrad cell for two years prior to being seconded to the field-executive pool, sent out, and thereby becoming fair game for anyone who could pick him up and break him open to get the information on the Leningrad cell.
Bluntly: I don't see how this could ever happen.
Let's look logically at the situation. If the Leningrad cell's as good as Q says it is (and others, like Tilson and Croder, seem to agree), then it would be a pearl without price in the intelligence repertoire of the British (confirmed by Croder's saying the CIA was "furious" over the Shapiro situation). They would guard this little nest of espions like a mother eagle guards its eggs. Even if "someone" seconded him to the active pool, a professional like Shapiro would immediately know the danger and (unless he had another reason, which I'll get to in a moment) would decline the assignment. Given the security-obsessed hierarchy in place at the Bureau, though it might be conceivable that someone might have assigned Shapiro to go out, somebody in Signals, or Briefing, or Documents would simply have to have noticed his former status and said something.
What are the alternatives to explain this debacle? To my mind, there are three. First, it might be that Shapiro possessed unique qualifications for some other mission which were held to outweigh the risk if he were to be captured. We know, for example, that Shapiro is Jewish and that he had contacts among the Jewish dissidents in Moscow. It is possible, then, that the Bureau (or some part of it) needed him to make contact with the dissidents in the context of some other mission. Alternatively, someone might have wanted to make use of Shapiro's knowledge of explosives for a specialized mission (he's certainly depicted in the novel as having an extraordinary level of expertise not likely to be shared by some other exec). It could even have been a combination of his contacts and his technical expertise (rather like Q being a pilot and fluent in metropolitan Russian in TSE) that made him, in someone's mind, ideal for a particular operation.
Second, given his rather passionate political views, I could easily envision Shapiro having actively campaigned (overtly and covertly) to be posted to Moscow (this is brought up obliquely in Chapter 17, where Bracken claims that Shapiro applied for an a-i-p position and speculates that he might've faked his whole capture in Hannover simply to get the Bureau off his back; the problem, as it always is, comes down to determining whether or not the Bureau is telling Q the truth at this point or when he was sent out, or at all). Incidentally, tangential support for the "faked-capture" theory might be found in Steinberg's having become uncomfortable with Q's probing questions in that interview in his office in Hannover, perhaps meant to suggest that Steinberg had been in on the whole affair (membership in Amnesty International, sympathy for dissidents, so forth).
Anyone brilliant enough to have come up with the idea of having several alternative life scenarios that he rehearsed daily down to the subconscious level, in case he were ever captured and questioned, would probably have had no trouble maneuvering himself into the field exec pool. Moreover, it's likely that anyone with that kind of confidence would've also believed that his subconscious rehearsals would keep him from being "broken," as indeed they apparently did. As it turns out, it was Croder and the other "nervous nannies" at the Bureau who were wrong and Shapiro who was right: he didn't break and he didn't give away anything, despite what they did to him. (Another possibility, that Croder fed Q the "second-pool" story just to get him to go out, when in fact the real reason for disposing of Shapiro was something else entirely, will be taken up in a subsequent TOTD.)
Third, it might have been sabotage, with someone in the Bureau working against its interests or (more likely) some Bureau employee deceived by another person, outside the Bureau, into making the "blunder."
Various combinations of these alternatives, of course, also suggest themselves. An opposition agent, having infiltrated the dissidents, might've signaled the Bureau for help (like the "Die Zelle" faction did in TSP) in the hope that someone like Shapiro, secretly known to have functioned in the Leningrad cell, would be sent out and subsequently blown. A dissident faction might've secretly maintained contact with Shapiro (this too is suggested as a possibility in Chapter 17) and then been exploited by the KGB, who are depicted as having successfully infiltrated the dissidents to an alarming degree.
Obviously, given the parameters, the problem becomes very complicated. One thing, however, I can't accept: seconding Shapiro to the field exec pool could not have been simply a "blunder." That Q thinks so is perhaps more due to disinformation from Croder than to Bureau malfeasance.