For example:
It happened only a handful of times in my lifetime: once when I went to the store. It just had to be done. There was no choice. Another when heading to the beach in 2013...
Should I just leave out the colon and use a period instead?
For example:
It happened only a handful of times in my lifetime: once when I went to the store. It just had to be done. There was no choice. Another when heading to the beach in 2013...
Should I just leave out the colon and use a period instead?
When a colon introduces more than one sentence, capitalize all of the sentences. For example:
To get rich quick: Invent a time machine. Use the machine for day trading. Beware of future versions of yourself (and other stalkers). Profit.
Some style guides (for example, AP but not Chicago) recommend capitalizing any complete sentence that follows a colon.
In your example, capitalizing “Once, when I went to the store” would clarify that you intend it as an (incomplete) sentence.
If the "once" is an example, it should be introduced with a comma rather than a colon. If you want to list all the cases, a colon would be legitimate, e.g., "It happened only a handful of times in my lifetime: once when I went to the store, once when I was conga dancing, and once when I was watching Plan 9 from Outer Space." "Once when I went to the store" is a subordinate clause, not a sentence, so the whole thing is one sentence.
The sentences following "to the store" aren't relevant. You aren't setting up a series of occurrences with them.
This reads like fiction, and so you have more creative license. I would word the passage like this to capture the list and the parenthetical within the list.
It happened only a handful of times in my lifetime: The first time, when I went to the store--it just had to be done; there was no choice--another time when heading to the beach in 2013...
I would use the em dash instead of parentheses to keep with the informal tone. Per The Elements of Style:
"A dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses."
Typically, when a colon indicates a list of phrases that can be standalone sentences, the clearest presentation is to separate them with semicolons. Additional color (it just had to be done. there was no choice) should be set off with parentheses or em dashes to indicate that they're part of the same thought.
It happened only a handful of times in my lifetime: once when I went to the store (it just had to be done—there was no choice); another when heading to the beach in 2013...