Speaking of bizarre, I encountered something totally bizarre yesterday. It's resolved, but it's SO insanely weird I wanted to post it here and see if anyone had any thoughts...here goes...
Our teachers are directed to My Moodle. One of our teachers was unable access a class because it was not listed as one of her courses, even though she was assigned as teacher. This class is one of four in a course subcategory (several category levels down from the Top category); each class being Quarter 1, Quarter 2, etc. Everything looked fine in Q1, the one alerting us to the problem, likewise for Q2 and Q3. (I noticed there were also other classes that weren't appearing for her, but I'll get to that later).
For the Q4 class, however (henceforth 'History'), when I clicked 'assign roles,' instead of the assign roles page for that class, what appeared was the assign roles page for a resource (a PDF file) in an entirely different class (henceforth 'Math'), several categories away. 
OK - so I go to the Math resource, everything looks fine. There are no locally assigned roles, yada yada. Just below that there is an upload assignment. I had no reason to, but I looked at it anyway. Noticed there was a bad image link in assignment description. HTML code showed the image source was pointing to the PDF file in the preceding resource. I don't now if the user was trying to link to the PDF or simply pointed to the wrong file for an image. Nevertheless, I removed the image reference.
Looked at some stuff in the database. I found some 'pseudo correlations' (more in a minute), but changed nothing and basically just got more confused.
So I go back to the History class, to assign roles again, to see if I could glean more information, and everything is cool! 
So then I log in as the History teacher who was having the problem, and all it cool there too! All the History classes appeared - as well as the other classes that weren't appearing - which incidentally, are listed below the History category.
Ok - back to the database - I noticed the assign roles page for History Course pointed to assign.php?contextid=627. In mdl_context the entry is: id 627, contextlevel 50 (course), instance id 55 (History). The path (whatever that is) is /1/24/269/287/606/627, depth 6.
So then I look up the locally assign roles page for the Math resource. In mdl_context it's: id 6591 context level 70 (module), instance id 2309, and the path is /1/24/267/193/2514/6591, depth 6.
I also found in mdl_context an entry for another assignment in the Math class: id 2008, context level 70, instance id 627, path /1/24/297/1896/95/2008, depth 6. This assignment however, was not related to the PDF resource or the assignment that tried to link it as an image. All clear as mud.
None of that probably means anything, and numbers are, of course, going to repeat themselves in different fields, but I was trying to figure out why a bad image ink in an assignment description was causing a roles issue in an entirely unrelated class; why the system no longer recognized the user as a teacher in the categories listed below the inflicted class; why all classes in that subcategory were also effected; and why that class in particular was the target in the first place. 
BTW - in case you're wondering - it wasn't me having flashbacks. I didn't notice the problem, someone else did. I just fixed it by some insane stroke of luck...