The horizon in the evening shot of the Opera in the Copenhagen habor is not perfectly level. The right side is lower the left.

Here the issue is fixed by rotating the image a tiny bit anti-clockwise.

Again the horizon is not level. Are you also a nerd who can not help noticing this problem?

Here the horizon is more level. Flying a paraglider in Australia can be amazing.

Again: does the sloping horizon bother you?

If not, forget about it, but if it bothers you, it is very easy and quick to fix in Google Photos.

This example is also related to the perspective issue discussed on the next page. Because this building is a lot taller than me I needed to tilt my phone upwards to fit the entire building in the frame. A consequence of tilting is getting leaning vertical lines. Notice, how the red line is leaning inwards.

In an attempt to neutralize the first red line I rotated my phone. I successfully got the vertical lines to the left aligned nicely to the frame side, but look what happened to the horizon in the back!

Rotating your phone is clearly not how you should fix leaning vertical lines. Instead, you need to move back and up to improve the perspective. You can read more about this in Plumb vertical lines.