A toaster such that both sides of the bread are not toasted at the same rate (this term could also be used to describe toasters that do not toast equally in all slots).
On my toaster, I believe this occurs due to an overcompensation by the engineers. The toaster holds the piece of bread by moving the outer grills towards the centre. This results in both pieces of bread being closer to the inner coils. Perhaps in an attempt to remedy this inbalance, the outer coils get significantly hotter (this can be verified visually). Unfortunately, they get so much hotter that the outer side of the bread toasts faster than the inner. In order to rectify this, I am forced to flip each piece half way through the toasting process.
On the other hand, some brands of toaster claim their lack of balance is a feature, not a bug! They give you a control to switch into "bagel mode" whereby only the inner coils are activated. This allows you to slice a bagel and only toast its soft innards. Myself, I just turn my bagels to face out.