When you go to use an expanding foam gun that hadn't been used for a while and only a disappointing little worm of foam comes out, don't be tempted to take the adjustment knob off and pull the rod back manually even if you've already taken the can off, unless you're ready for a high-pressure eruption to cover everything small gobs of foam.
Fortunately, I went outside to take the can off just in case, but even so.....
Ah well, I probably saved the cost of the new one many times over by not chucking away 90% full cans of hand held foam that had gummed up.