Quick reflex/lenses care-taking trick

How many time has this happened to you?

A few times for me. And I'm giving the solution also the the issue you face when the lens is not working fine when focusing. Often the problem is simple: oxidation or dirt/dust on the contact. Also the key to resolve this kind of problem is at hand-reach.
The answer is: oil, preferably extra virgin. Natural anti-oxidant par-excellence!

Use a soft rag, and be careful that it's a  rag which won't leave behind pieces of paper/bits of itself. I personally use a google wipe and it works perfectly. Clean the contacs-area and if needed leave for a 10-20 minutes the wipe on the contacts to let the oil work. After that, clean up with paper(again not leaving bits) and try...no more Error 99 ;)