It isn't clear in the demo visualization, but insertions into the Cuckoo filter do get rejected after exhausting a maximum number of kick/relocation attempts. If I find some time I'll make that more apparent.

As for false negatives, this can happen in both Cuckoo and Counting Bloom filters if a value that is never added is removed, and the value would have resulted in a false positive on query. The contract for removal is that you must know that the value is currently in the filter. I'll clarify that in the text.