A fascinating video of a UFO flying across the sun was allegedly taken in Mexico. The video has subsequently been pulled down. However screen captures were taken before it was too late. Was it a UFO, a flying saucer, or a cloud? Don't know. Here is the brief analysis I did. Original Image:

Inverted Colors:

Larger Size - note color of clouds and color of object very similar.

These are other images of clouds in front of the sun, chosen using Google Image Search for near-identical images.

Lenticular Clouds amazing saucer shaped.

It's hard to decide what this really is. Additional: I played with the contrast and got these:

The shadowing under the "flying saucer" could be an artifact of software.