When undoing toyota head bolts - please pay attention to the bolt head. I am making this thread after I was asked a similar question on email.
The question started as "why do toyota give such soft headbolts that their heads strip out".
The problem was user error. Toyota use a 12 point bi-hexagon female socket headbolt design. This is not interchangeable with the more known XZN design which also has 12 points but is actually called a triple square. The guy went out and bought an XZN drive and got himself into trouble. The double hex bits are a bit hard to source.
The triple square bit will fit in the bolt head but only the edges of the teeth will engage the bolt splines which will cause failure when torque is applied. To make it more clear a triple square socket has 3 squares super imposed on each other while a bi hexagon is 2 hexagons superimposed on each other.
The internal angles are different of the tools as a square has 90 degree corners while a hexagon has 60 degree corners.
Use the correct tool for the job - If you are using a mechanic then buy the proper tool yourself and have the mechanic use it. Don't let a simple tool make your day miserable.