I have an android phone with the SwiftKey keyboard app (so do swuillions of people, so this doesn't identify me much).
I don't know if it's an Android setting or a SwiftKey thing, but I have just noticed that the clipboard retains text I had copied in the past, and this does not get cleared after a reboot.
Like many, I write a draft of my reviews on the phone then post it. In some apps, a long press brings up only "paste" as an option. In others, it brings up paste and clipboard. I clicked on clipboard, and I could see shit I had written weeks ago! Of course I deleted it straight away! SwiftKey also has a clipboard menu.
Also, remember to set your keyboard to incognito. This is easy to do with SwiftKey and it means it won't remember the new words you type. Imagine you're writing a text to your OH and the autocorrect brings up a dodgy word that might reveal your hobby...