In another post today on this very forum, I was on the verge of using the following construction:
"If you happen to [do X], could you please [do Y]?"
After a moment's reflection, I had doubts about the use of "could" in this context and looked up the use of "happen to" in conditionals in L. G. Alexander's Longman English Grammar. The construction is covered under 14.8 and is never shown with "could". Instead, Alexander stresses the use of the imperative, as in:
e.g. "If you happen to see apples at the store, pick up five for tomorrow's picnic."
It is ungrammatical, though, to use a question beginning with "could you..." after a conditional clause?