The PC's keyboard layout is just different enough from the Mac's to make me trip up from time to time. The most obvious difference is the Mac's use of "Cmd" as the primary modifier key as opposed to "Ctrl" on the PC. I'm used to hitting "Cmd" to the left of the space bar, rather than "Ctrl" at the lower-left corner of the keyboard. I imagine I'll soon get used to it, but if things get too dire I can always break out SharpKeys to remap those keys. I could duplicate the Mac's key order by reassigning what the "Alt," Windows, and "Ctrl" keys do when pressed.
I've also had to relearn how to insert special characters with keyboard combinations. Macs offer persistent, system-wide shortcuts for foreign-language accents and tildes (for example, pressing Alt + n, then n again, gives an n with a tilde over it). Windows offers similar functionality (for example, ctrl + :, then u, gives a u with an umlaut over it), but those shortcuts don't seem to be consistent across the whole OS. Neither LibreOffice nor Google Docs, two popular word processors, seem to support these combinations, although Microsoft Office does. It would be a shame to have to memorize Unicode characters, or mess around with the "insert special character" palette, just to type accents when I need them.