The best play, shown as 6/off, is in fact the perfectly legal 6/4/off.

What is arguably a bug is the way gnubg displays it. This kind of elision is standard when the same checker moves twice (for instance, opening with a 65 : 24/13) but in this position it is indeed confusing.

Fixing it is not entirely obvious. Always displaying all the moves (24/18/13, etc...) is awkward and not what people are used to.

Finding rules that indicate when no to do it might be tricky. When there are wasted pips in a bearoff, as here, seems a good start, but it may not be always necessary, nor cover all cases.

Release 1.06, Windows 10

The game allows an illegal move, and even suggests it as the best play. We came across it during play, but it can be reproduced by setting up the board and dice as shown in the attached image.

The dice are 6 and 2, but it allows a play where only the 6 is used. There are two ways to use both the 6 and the 2.