Sleeping in the same bed when the lover is around.
A child who sees the adulterers mid-WooHoo may cry and attract the attention of the cheater's spouse.
However relationship drop won't be as large.
A lover sees the flowers a date may have left (though an apparent bug in The Sims 2 means flowers sitting on a shelf or other surface might go unnoticed).
Going on a date when the other lover enters the room.
However, in The Sims 2, a Sim doing this will not be caught if the other lover is sleeping.
WooHooing as another lover passes by on the same lot, even if they're on opposite sides of the lot.
Having an affair can be risky, as getting caught can possibly ruin the relationship with the other Sim.
3.1 The Sims, The Sims Bustin' Out and The Urbz: Sims in the City.