In S3E1, Bonnie's mother said that she was dying because of her heart. In this episode it is said by her son that she died of cancer.
Bonnie is at her mother's grave just days or weeks after the death. However, the grave is flat, covered with grass, and has a headstone present. None of these things should be true in such a short time period. Graves takes weeks to settle. The same is true for grave stones. They are usually dropped off by the monument company and left for the cemetery to set in place, either flush with he surface. Or leveled and plumbed in the case of above ground level monuments.
In a first season episode, Christy is in a situation where she has to add two big numbers quickly. Regina and Bonnie are in the same room and do the math in a flash. At that time Christy say's "I forgot drug dealers are really good with math". This detail was forgotten by this episode when Bonnie couldn't add two four-digit numbers together.