Some owners would notice coolant loss and leaks on the back side of
> the 2.4L engines, resulting in $4000+ of repairs, because head bolts
> simply come loose by themselves. I suspect 2007+ engines will suffer
> the same fate down the road.
>
> "3/6/2008
>
> Coolant Loss on 2002-2006 Toyota 2.4L, 2AZFE engines
>
> The following information concerns the loss of coolant on some
> 2002-2006 Toyota 2.4L, 2AZFE engines. The location of this condition
> has been noted at the rear of the engine where the cylinder head bolts
> to the cylinder block.
> It has been reported that stripped head bolt threads have been found
> on some of these engines during tear down. When the two rear head
> bolts lose their clamping ability they allow coolant to seep out at
> the rear of the cylinder head.
>
> To repair this condition, engine builders report using thread repair
> tooling used for various GM engines. Specifically, Cadillac 4.6L
> engines use the same threaded head bolts (Figure 1). The head bolts
> for this Toyota engine may be reused if they are not damaged and meet
> the following length specification.
>
> Using a vernier caliper, measure the length of the head bolts from the
> seat to the end.
>
> Specified bolt length: 6.350½ to 6.465½ (161.3 mm to 164.2 mm). If
> the length is greater than the maximum, replace the bolt.
>
> This information was provided by the Engine Builders Association > (AERA)."
>
>formatting link
How did that happen, it seems they were over torqued when being made?