03.21.08 - "Suprise Party Disadvantages", as described by Wikipedia and illustarated by Vans Skate Overlord, Robin Fleming:

There are some disadvantages to this method of birthday celebration. For example, the honored person may feel ignored because their birthday is not mentioned until the party occurs. Also, if the honored person does not appreciate the surprise effect, they may become angry or disgruntled. In addition, the honored person may not be appropriately dressed for the occasion, and may feel out of place in frumpy, casual clothing compared to the other dressed guests. Finally, the sudden emotional stress caused by a surprise party can cause stress cardiomyopathy.