I consider myself to be a fairly proficient user of English as a second language, but there are some things I will never get right:

• Using a “W” when a “V” would do (like in woodoo voodoo; and yes, just like Chekov).
• Pronouncing “iron” correctly.
• Understanding whether “substituting X for Y” means that you used to have “X” and now you have “Y” or vice versa.
• Using em-dashes without—yes, without—any surrounding spaces.

The first two are entirely my fault, the third runs contrary to most other languages, but the last one is just dumb and that rule should be abolished.