Don’t write the -ed and -ing forms of occur with only one ‘r’. The correct spellings are occurred and occurring:
✗ It never ocured to me that I could be wrong.
✓ It never occurred to me that I could be wrong.
✗ Many changes are occuring at every level of education.
✓ Many changes are occurring at every level of education.
The reason why the ‘r’ is doubled in the -ed and -ing forms is that the stress falls on the final syllable of the verb in its infinitive form: occur.
The same rule applies to most other verbs ending in -ur and -er where the stress falls on the final syllable:
When the stress does not fall on the final syllable, the -ed and -ing forms are spelled with one ‘r’: