The original thread was for a bug in 5.0.x, fixed in 5.1, but it would be a very serious issue in 5.5. It basically means that the local memory of a beta node has been assigned to an alpha node, or that the wrong memory had been retrieved. In any case, the rete is broken: wrong facts or types would be joined in a different order or way than stated by the rules.

Is this something you're experiencing , or just worried that it may happen?

Causes could be an invalid rule in the first place (running the engine without checking for compilation errors), a bug in the construction process (unlikely, unless the rule is a very rare edge case), or a complex scenario where you add and remove rules dynamically from a knowledge base.