If you have added a P-trap to your bathroom so that the tailpiece extends beyond the trap, then you may find that you have a few problems. You may find that the two pieces just don't fit correctly into each other, and this means that any problems you have are sitting in the bathroom.
1. The Plumbing Fails
The biggest problem that can occur if the P-trap and the tailpiece don't match is that your plumbing will fail. The two pieces will not hold together, and the force of water on the inside can simply knock the connections away. When this happens, it can cause major flooding in your home. This can mean you will have to call in an expensive emergency plumber.
2. Slow Leaks
Another problem when the connection doesn't quite match is a slow leak. This is like a plumbing failure, only rather than an explosive breakdown, the pipes just slowly drip water onto your floor. This can result in a permanently damp floor which encourages mold and an unpleasant smell in the bathroom.
3. Water Doesn't Flow
The other problem will be that your water simply won't flow correctly. This means that your water is backed up in the P-trap and remains there, causing a smell.