If one eats a non-mezonos food prior to a meal in which he intends on washing, he may be obligated to make both a bracha rishona and bracha achrona on that food even if he will have a full meal and bentch following the bread-meal. These halachos come up often at weddings bar mitzvos sheva brachos and other parties when eating at a shmorgasboard prior to washing for the meal. If the food eaten before the meal, which would require a separate bracha during the meal (e.g., fruit) is eaten with the intention of exempting the person from making a bracha when eating that food during the meal, then he should make a bracha rishona but no bracha achrona because the birchas hamazon will cover the food item eaten during the meal along with the one eaten before the meal. The eating before the meal will be considered part of the same meal. However, if you do not intend on eating that food during the meal, then you must make a bracha achrona on the food eaten before the meal prior to washing because there is no connection between that food and the meal (Mishna Brura 176:2).
If the food item is one that otherwise would not require a separate bracha during the meal because it is a food eaten as part of the meal (e.g. chicken or vegetables, as opposed to dessert items), then even if one intends to eat the same food during the meal, the food eaten before washing would not be considered part of the meal and therefore require its own bracha rishona and bracha achrona prior to the meal (Mishna Brura 176:2).