As for the mechanics of -why- this happened... the ~ command toggles capitalisation. Now, when you hit PgDn within a vi that doesn't handle it (such as on any Sun), you'll send something like ^[[4~, where the ^[ is the escape character. This turns off insert mode, puts you into command mode, beeps on the [4, and sends the tilde key, toggling caps for the letter under the cursor.

vim of course handles these keys properly in insert mode!

(Update: corrected [4 sequence)

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Me spell chucker work grate. Knead grandma chicken.