I'm a native speaker of Mandarin Chinese from Taiwan. Some people often mix up 在 (zài) and 再 (zài) in writing. It's a bit hard to explain their definitions since they are merely function words (words that have little lexical meaning and express grammatical relationships among other words within a sentence), so I'm just gonna copy and paste their definitions from an online dictionary:

在: to exist; to be alive / (of sb or sth) to be (located) at / (used before a verb to indicate an action in progress)
再: again; once more; re- / second; another / then (after sth, and not until then) / no matter how ... (followed by an adjective or verb, and then (usually) 也 (yě) or 都 (dōu) for emphasis)

As you probably have noticed, their meanings don't overlap at all. The only reason some people mix them up is because they are homophones.

Another typo some... let's just say, less educated, people often make is 因該 (yīn'gāi). The correct word is 應該 (yīnggāi), meaning should; must. 因該 is never correct. You can think of 因該 as the Chinese version of the much dreaded "should of." The reason is that the distinction of -in and -ing is slowly fading away in Taiwan (it is still very much thriving in other Chinese-speaking societies), and some people just type too sloppily to care.

By the way, I should mention that 在, 再, and 應該 are very basic words, probably one of the first 500 words a non-native speaker learns.