Rapid flashing turn signal...
What can happen that will cause your problem is that the new bulb didn't make contact with the shell of the socket, which goes to ground. So the bulb will light through the parking light half, which, if the parking lights are on, has +12V on it. This makes the flasher "think" that the bulb is blown, and it flashes at double speed if the lights are on. If this is your problem, you will see that the front bulb is lit when the rear bulb is off, and vice-versa. The solution is to clean the socket and /or repair the ground circuit. I ended up replacing all the sockets in my '74 280C because they wouldn't make good contact. I epoxied new sockets in the old plastic housing, and then soldered the wires to the original socket. This fix worked very nicely. One trick that all the new cars use nowadays is to fill the sockets with silicon grease before inserting the bulb. This keeps the sockets from corroding and prevents water from contacting the metal, should it leak into the housing.