Cant track down a specific cause but I am experiencing the following (twice in the last ten mins)
1) You lose the ability to DRAG items from the bars to the project.. i,e can drag an item into the wizard box, and also could not drag a Foregin Key onto a table
Clearing down browser totally and re logging onto Builder shifts the issue
2) Open a layout results in a small tab as though the layout is open, but no content. This tab cannot be closed down until you actually close the TABLE and you can open another version of this layout. I have not been able to reliably replicate this issue, though it was happening consistently at one point