I made a starter with WPL001, 1150ml water + 115g DME, boiled it down to 1L after 10 minutes, hit 1.039 and put it on the stir plate overnight at low-medium speed. This morning I noticed what I thought was a slight overflow on the side of the flask ended up being a thick, gooey sludge underneath the flask. Is it safe to assume that what spilled was the crux of that starter, i.e. the yeast, and that I’ll need to start over?
I don’t have a complete answer to your question, but I have experienced this a couple times before. I went ahead and used the starter both times with successful outcome. So, while I don’t think you need to start over from personal unscientific experiences, someone may overrule me with a better answer. 
I also don’t think that you have to start over.
Kai
This has happened to me several times with 001. It is fine. Brew on!
After resting in the fridge overnight, a solid 1/4" of cake has settled on the bottom, so I suppose I’m all set. Thanks for the reassurance.
If you really want to be assured you’re pitching properly, take a liter or so of wort from your brew day and put it on the yeast for 4-6 hours on the stir plate prior to pitching in the full batch.