The correct article to complete the sentence is 'an' because 'unusual' begins with a vowel sound, requiring the indefinite article 'an' for a smooth transition.
The sentence requires an indefinite article before the noun phrase 'unusual thing,' since it is the first mention of that thing and it's a singular, countable noun.
The appropriate article to use in this context would be 'an,' due to the vowel sound at the beginning of 'unusual.'
Therefore, the completed sentence should read, 'An unusual thing happened at work today.'