Time to inspect your stock MGA steel wheels and check for cracks. The Stock MGA wheels were spec'd and built in a time when bias ply nylon cord tires were the norm, radial tires were just struggling to make inroads into the world, and no one had hardly dreamed of what we know today as modern race tires. The MGA wire wheel is (by today's standard) somewhat thin and flexible. With good modern tires and some spirited driving it is possible for one of these wheels to develop a stress crack.
First time I've seen this type of crack, this time in a wire wheel rim. Crack is between the two red dots. Photo supplied by Paul Acto in Barrie, Ontario, Canada.


Addendum Sep 14, 2018:
Mitchell Andrus in Mills River, North Carolina, USA wrote:
"Found in my rejects pile. I'm guessing a spoke or two were just a little too tight".


On Oct 13, 2018, Jim T in Lake Zurich, IL, USA wote:
"Just something to keep your eye out for... WOW. Never thought I would see this. Hard to see until wheel was off". -- Later, Jim