This page exists to collect the strange behaviors seen in Mystcraft. Some of these may be bugs in the current version; others are intended oddballs to be aware of.
In general: if it seems counterintuitive at first, it should be listed here.
Transparent blocks and outdoors
Any block that permits light to travel through (including torches, fenceposts, and glass) will permit instability affects to pass.
This is probably not intended. A switch to testing for "can lightning strike here?" or "do any of the blocks above me have a 'you cannot walk here' size of a full block?" would probably be an improvement.
The current version makes it very hard to generate instability just because you left stuff out of your writing. However, there is no way to predict ahead of time how the grammar will expand incomplete writing, and what is considered "incomplete" is often very surprising. This is known to be intentional.