Threads and UI

Threads and UI
I have a feeling that sometimes, something, somewhere is trying to UI things from a non-UI thread. Naturally, this is something we strive to avoid - but with all the injection of handlers - and literally hundreds of places where someone could think "surely, it would be clever to update that status label now" - it is a bit of a grunt task to review all the code.

So, before embarking on such a review quest, my question is:
Which symptoms could arise from touching UI from a non UI thread?
AV? Deadlocks? Temporary freezes? Donald Trump?

I am seeing temporary freezes (20+ seconds) in the UI. The background threads are all looking active, doing and logging their stuff.

My first thought was a lock contention - but I log all locks that fail - and the log shows nothing of the sort, so I am in a bit of a pickle here.