I don't know what, but something causes it to leak. I've been using it for about a year and several of the low numbered containers have accumulated duplicates:
Edit: Apparently HN strips box drawing characters. Imagine the output of `sparklines 6 15 7 5 2 1 1 4 3 1 1 1 1 1 1 1`.
I can confirm what others are saying, using temporary containers with the default settings left me with hundreds of containers to clean up.
I don't remember what the default settings are, but I do remember changing a fair number of them (and they are numerous). Definitely not the most friendly tool to casual use. But when fully configured, it is my second favorite addon (behind uMatrix).
Are you sure you didn't accidentally change it?