Were there any WWE moments when somebody accidentally hit his opponent in a match?

They're always hitting each other.

The technical term is being stiff.

Apparently in Japan they are very stiff, as was the wrestler big bad Vader.

These are the injuries Mick Foley has suffered in this “fake” sport called pro-wrestling:-

Six concussions from 1986 to 1998
- One broken jaw in 1986
- Two broken noses in 1993
- One broken cheekbone in 1998
- Lost four front teeth from 1989 to 1998
- Two-thirds of his ear ripped off in 1993
- A separated shoulder in 1990
- A fractured left shoulder in 1989
- A dislocated shoulder in 1998
- Second degree burn on his shoulder in 1995
- Second degree burns on his arm in 1995
- 54 stitches on his left arm in 1995
- A broken right wrist in 1989
- Bone chips in his right elbow in 1996
- Six broken ribs from 1991 to 1998
- A torn abdominal in 1992
- A torn ACL
- A broken toe in 1991
- A total of over 300 stitches in his arms, head, eyebrows, hands, ears, shin, cheek and lip
Thousands of thumbtack holes

Here's some pictures from those matches

This last one is joey Mercury after a ladder match against the hardy boyz after a see saw move with a ladder.

Injuries happen and accidents are going to happen when 2 well built athletes throw each other around the ring but because the outcome is predetermined the physicality and violence can be all to real.