Final answer:
The situation when part of a building is inadvertently built on someone else's land is called an encroachment.
Step-by-step explanation:
When a portion of Wendell's building was inadvertently built on Ginny's land, this situation is referred to as an encroachment. An encroachment occurs when a structure, like a part of a building or a fence, extends beyond an individual's property and intrudes onto adjacent land without the landowner's permission. Encroachments can lead to legal disputes between neighbors over property rights and boundaries, and usually, the resolution involves negotiations or legal action to address the trespass.