[TrianguloY]

That's the pen I always used at exams at university! Maybe at bit risky, but never had any issues (and being able to fix things without using that white thingy to erase was wonderful).

When a friend discovered them he used it to do a puzzle (for a home scape room). It was a crossword than when solved it said something like "heat this", and when you did it, the black squares disappeared revealing the code. The squares were made with that ink.

[semi-extrinsic]

A friend of mine had an incident with these pens in high school. Used one on his physics midterm exam, teacher put a laptop blowing hot air onto the pile of papers before grading, so 3/4 of every page were just gone. He had to retake it.

[TheKnack]

If you put the paper in the freezer, the ink will reappear.

The ink doesn't really go anywhere, even when you use the "eraser". It just becomes transparent. This can be reversed by exposing it to cold.

Pilot mentions this on their tips and tricks page: https://www.pilot-frixion.uk/uk/tips-and-tricks/

[colordrops]

But if you erased a lot then the page would be a mess after everything re-appears.

[analog31]

Wow, that's just like the secret spy pen from the Johnson Smith catalog. Brings back memories.

[dqv]

That’s too bad. He could have just put it in the freezer to get the ink back. Uh, but let’s keep that between us.

[CamperBob2]

Spoiler: that was the physics test. He failed.