I ran into quite a bit of contamination last week with at least 4 grow blocks.
The big mistake I did was bulk sterilize about 12 bags in a couple days and wait for several weeks before using them. They hold up really well for up to maybe a week or two, but they will naturally contaminate as a little air seeps through the folded opening at the bottom of the bag.
My lessons from this situation:
1) In a home-based growing environment, anything that was once thought of as sterile will eventually become unsterile. There are too many contamination vectors outside of a lab to count on that jar you sterilized a few weeks ago to still be clean. If it's been sitting around for a while, just re-sterilize it.
2) When you need to use something sterile, sterilize right before using the thing, e.g. you need a grain jar, so you should sterilize the grain jars the day before, and keep it in the sterilizer until you're about to use it (let it cool off overnight, of course). This also applies using syringes: Flame sterilize before every use.
3) For grain jars, check your lid gaskets, fresh air exchange patches, and syringe ports. These are parts that are consistently poked, pushed, bumped, squeezed, what-have-you, and will eventually wear out and expose the grain to contamination. Get those parts replaced on an occasional schedule.
I once made the right call to re-sterilize my grow blocks after not using them for a while. This time I didn't, and now I've wasted a good amount of Lion's Mane grain spawn. I learned my lesson!