The bug

When comparing two dragon head blocks with /testforblocks, if either of the blocks have ever been powered, the comparison fails, even if the blocks are identical.

How to reproduce

1. Place two dragon heads, ensuring they are facing the same direction.
2. Place a redstone block next to one or both.
3. /testforblocks <block1> <block1> <block2> all

Expected result
Success message: 1 blocks compared

Actual result
Error message: Source and destination are not identical

Cause

Dragon heads save a timer to NBT. As a result the animation that should be entirely client-side is implemented on the server. See also MCPE-152603 and MCPE-113669