For example I've placed the piece NorExSmFree_01 in my dungeon,a Nordic Dungeon piece with a door on a straight 90 degrees wall,and none of the other walls in the Nordic set is straight,so I can't align this piece with any other Wall piece,as if I place other kinds of walls, there will be large cracks from which the player will be able to look at the void.
Some times I realize that a specific part doesn't have other parts that align well with it,much later,when I have already built a large portion of my level based on that piece,meaning I'll have to scrap hours of work and delete many pieces and start designing all over again.

I'm not expecting a solution to this other than prepare myself with more patience,but it hit me so much emotionally that I had to scrap so much time of work,that I wanted to share this.