If you generate an old world and make it infinite, there'll be generated a weird landscape which doesn't fit to the old at some points. It looks like a bug of flat worlds in earlier versions of the pc version. At some points the landscape fits completely to the old, but sometimes it is higher than the old and has got other biomes. At a few cunks it fits, and at other not.
Here is an example for regenerating this bug:
1. Type the seed 1927496 and select world type "Old".
2. Make the world infinite.
3. Load the new world and look for landscape bugs as shown in the pictures.

The pictures, in which there is written "Before" in the chat, were taken before making infinite, th ones, in which is written "After", were taken after making the world infinite.

WARNING: THIS BUG WAS FOUND IN MCPE 0.11.1, BUT 0.11.1 WASN'T SELECTABLE WHEN GENERATING THIS BUG REPORT. SO IT WAS AN COMPLETELY OFFICIAL VERSION OF MC, NOT A BETA!!!!
0.11.0 Was released on june 4th, but the 'Create Issue' page wasn't updated since 0.11.0, 0.11.0 is registered as an unreleased version, 0.11.1 didn't exist.