Sentry and century are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Sentry is a noun. It means a soldier or guard working at a gate or outpost, monitoring who enters and who cannot, and defending their post if necessary.
Century is a noun. It means a period of one hundred (100) years.
The following story uses both words correctly:
Seneida was frequently appointed sentry duty at the base’s main gate. It was probably the most boring task in the armed forces. She had to remain alert. It was hot, and it was very dull marching in the same pattern over and over again. Each second felt like a year had passed, so by the end of her six hour shift, it felt like she’d been standing for a century.