I have a 1990 acura integra 5 spd. Some how i ended having transmission fluid in my coolant reserve box. what?

just got an oil change and right after i got home i checked under my hood and there was transmission fluid in my coolant system. did mechanic did this to me or do i have a problem in my car. It a manual car.I have a 1990 acura integra 5 spd. Some how i ended having transmission fluid in my coolant reserve box. what?YOU HAVE WHATS CALLED A OIL COOLER EITHER INSIDE YOUR RADIATOR OR EXTERNALLY WHICH IS HOOKED TO COOLANT TO KEEP TRANS COOL 1 OF THE LINES IS CRACKED OR THE OIL COOLER ITSELF IS BROKE CAUSING TRANNY FLUID TO BACK UP INTO RESIVORI have a 1990 acura integra 5 spd. Some how i ended having transmission fluid in my coolant reserve box. what?Your transmission most likely has the transmission cooler built into the radiator so if the cooler or cooler lines are leaking that would cause this problem. as to if the mechanic caused this problem i cant answer that.