Once you head into a sentence, your choices are limited by the range of grammatical patterns in English. You cannot begin with one grammatical plan and switch without warning to another.
The writer began with a long prepositional phrase that was destined to be a modifier but then tried to press it into service as the subject of the sentence. A prepositional phrase cannot serve as the subject of a sentence. If the sentence is to begin with the prepositional phrase, the writer must finish the sentence with a subject and a verb.
The writer who wishes to stay with the original verb (double) can head into the sentence another way.
The adverb clause When an employee is promoted without warning cannot serve as the subject of the sentence. The revision replaces the adverb clause with a gerund phrase, a word group that can function as the subject.
The Although clause is subordinate, so it cannot be linked to an independent clause with the coordinating conjunction but.
Occasionally a mixed construction is so tangled that it defies grammatical analysis. When this happens, back away from the sentence, rethink what you want to say, and then say it again as clearly as you can.
Multilingual: although, because
Multilingual: Double subjects and other repetition
Exercises:
Mixed constructions 1
Mixed constructions 2
Mixed constructions 3
Mixed constructions 4