I can't say how many times I've tried to work out a complex substitute command in Vim only to have the pattern fail and not match what I wanted. I've recently come across an easier way to accomplish a substitution on a pattern that's already guaranteed to match. Say you have the following data in your editor.
"this is a line 12345"
Execute a search (instead of a sub):
" match digits
Now execute a sub with no pattern:
The result would be:
"this is a line 7890"
Obviously this is a simple example, but I find this technique handy whenever I'm working out complex patterns.