In 8-ball, if you sink the opponent's ball while sinking your own, do you keep playing, hand over play, or is it considered a foul?
Assuming that you have not committed a standard foul otherwise, sinking the opponent's ball is not a foul in itself, so my interpretation of it is that you can continue with the game as you would have normally.
The following is an excerpt from World Pool-Billiards Association rulebook for 8-ball:
3.9 Standard Fouls If the shooter commits a foul, play passes to his opponent. The cue ball is in hand, and the incoming player may place it anywhere on the playing surface. (See 1.5 Cue Ball in Hand.)
The following are standard fouls at eight ball:
6.1 Cue Ball Scratch or off the Table
6.2 Wrong Ball First The first ball contacted by the cue ball on each shot must belong to the shooter’s group, except when the table is open. (See 3.4 Open Table / Choosing Groups).
6.3 No Rail after Contact
6.4 No Foot on Floor
6.5 Ball Driven off the Table (See 3.7 Spotting Balls.)
6.6 Touched Ball
6.7 Double Hit / Frozen Balls
6.8 Push Shot
6.9 Balls Still Moving
6.10 Bad Cue Ball Placement
6.11 Bad Play from Behind the Head String
6.12 Cue Stick on the Table
6.13 Playing out of Turn
6.14 Three Consecutive Fouls
6.15 Slow Play
6.16 Ball Rack Template Foul