There are hazards to avoid with some knots.
The Slip Knot Dilemma:
A common practice is to use a slip knot to back-up a new climber. There are two outcomes to tying a slip knot. One can capture the climber's leg and make rescue problematic. The other slip knot can't form a cinch.
In this illustration, the top slip knot is a problem. The bottom one is OK. Tie them and place them on your foot and pull. The top knot will cinch while the bottom knot doesn't.
The Fatal Buntline:
If a carabiner is clipped into each of the buntlines, the one on the right will slide off the end of the rope and fail. By definition, the one on the right is a buntline, yet its use has a potentially fatal outcome.
There's a similar problem with the scaffold hitch.