Here's a list of bugs I've discovered so far, while messing around in the Mission Builder, in no particular order.
[BUG] Copied "Spawn Vessel" nodes fail to spawn crafts
If "Spawn Vessel" node is copied, it will not spawn the desired vessel.
For example, if you need two vessels in orbit, relatively close to each other, setting the orbit settings in one node, then copying it and change which vessel to spawn would be easiest. However, copied nodes will not spawn any vessels. You must create each node from scratch atm.
[BUG] "Time Since Node" node shows nodes docked to the start node
Nodes docked to the start node do not activate, thus it makes little sense to be able to select them for "Time Since Node" nodes, since it will never test true.
[BUG] "Spawn Vessel" spawned automatically with "Start" node has no "Focus on Vessel" option
The "Spawn Vessel" node has a "Focus on Vessel" option to focus on the vessel after it has been spawned.
This option lacks entirely from the one spawned by default in new missions.
While this option is implied for nodes docked to the start node, the option does not appear when undocking the default node, nor does it appear for "Spawn Vessel" nodes spawned explicitly and docked to the start node.
Either the option should be shown always, or appear/disappear when docking/undocking.
[BUG] Menu becoming inaccessible
I'm not sure when or why it happens, but sometimes when testing a mission, escape will not open the menu, and the only way to quit or get back to build your mission is to Alt+F4 and restart the game.
[BUG] Drop down for which vessel to spawn in the "Spawn Vessel" node does not render correctly until scrolled with the mouse wheel
[BUG] Activate Once Only should not be true by default
It is far easier to tick Activate Once Only on a single node, when you actually need it, than having to untick it on every single node, you create ever, when you don't.
[BUG] Activate Once Only is not preserved during copy
When you copy a node, Activate Once Only is re-ticked in the copied node.
I'll update this as I discover more bugs.