Yeah, see this older thread:
Beeminder sometimes does indeed end up requiring an extra item because of floating point issues. That’s not great, but at worst you can contact support to fix up your goal for you. In cases where it truly is the fault of a bug in Beeminder, this or any other, they absolutely will fix it manually for you.
(Also, if you’re willing to get more technical: as a cludgy workaround for this sort of floating point issue, the API, when checking that you’re not making your road easier, does so only to within a tolerance of 10^{-9}. You can take advantage of this hidden undocumented workaround to update your goal’s graph via the API to jump downwards on that day (from 51.00000000000001 to 51, which is a floating-point difference of approximately ~7.1 \cdot 10^{-15}, well below 10^{-9}), and thus fix it yourself. I think that would work also via the visual graph editor.
For many or most users just contacting support to manually fix it is probably easier, but given that this long-standing issue is unlikely to be truly fixed any time soon, it may be worth knowing that you can work around it with the small amount of leniency the API gives you.)