You weren´t paying attention then. The game routinely promotes workers to fill specialists jobs leaving old specialists unemployed. It isn´t 100%, but it happens often enough that i find myself preemtively microing the situation by disabling the building right as it is about to finish. This is a glitch one of many, but you would see that as 1-2 unemployed specialists sitting around for no particular reason. If there weren´t any glitches, the system would only be either tedious or meaningless, depending on what you were trying to do..
Never seen this myself. Doesn't mean that it doesn't happen, but it does mean that the shortfall isn't enough for me to notice.