Seems like a dodgy prefab, don't use it.

Why even use a table-prefab anyway!? How hard could it be to build a table yourself? It's just a flat surface supported by a few legs...

It's not necessarily the prefab's fault nor yours. Hammer is rife with bugs. There's no reason it should ever let you create infinite-sized brushes in the first place.

In my experience you just have to deal with it. Delete and rebuild.

But also try to pamper Hammer a bit and you'll be less likely to run into these bugs. Don't create extra-huge or strangely-shaped brushes. Keep them on the grid. Don't put them anywhere near the edge of the grid if you can avoid it. Make everything out of the simplest, most medium-sized pieces possible.