Somehow this piece has been around, unmarked, since 1977.
Eva Kraus, the museum director, said the damage was not permanent and would probably be relatively easy to repair. "We do realize that the old lady didn’t mean any harm," she said. "Nevertheless, as a state museum couldn’t avoid making a criminal complaint. Also for insurance reasons we had to report the incident to the police.... We will let the lady know that the collector took the damage to the work in good humour, so she doesn’t have a sleepless night," Ms Kraus said.Doesn't that wreck the work of art? It's conceptual art, a Fluxus movement thing.
The museum said that in future it would alter the label for the work to make it clear visitors were not permitted to fill in the blanks.

Based on that I say: Screw the collector. What was the artist, Arthur Köpcke, doing and what the woman's act mean to him? His Wikipedia page is not in English, but here's something:
In the mid-1960s Arthur Köpcke worked on a series of works that he called Reading-Work-Pieces. The individual ”pieces” consist of a wide range of materials, including picture puzzles, tests used within perception psychology, long philosophical texts, crosswords, and instructions for perfectly simple everyday actions. He painted the pieces in oil on canvas, adding cuttings from newspapers or magazines.So can we go for some depth here?
With his reading and training pieces, the artist wished to increase the spectators’ awareness of the systems, actions, and rituals that we persist in and carry out every day without reflecting on them. Behind the seemingly disparate and random materials, the cryptic statements, the subtly humorous tasks, and the banal pictures lies a deep interest in the functions and meanings of signs and sign systems....
