In the summertime, even with the best sanitation practices, it's possible to pick up an infection from the air while the wort is cooling. I am a sanitation fanatic, and I use an iodophor treatment for my fermenter right before I put the wort into it after chilling it down to just above room temperature. One summer, I picked up a lactobacillus infection in an American lager. I could find nothing wrong with my sanitation and cleaning practices, so I did some research and found that old timers often did not brew in the summer time because their beer would often pick up an infection. I made the same beer later in the year without any problems.
You can buy a product called Lysozyme here in the United States at least that will prevent lactobacillus from getting a hold in your beer. I may get some this year to use for my summer time brewing.
Lactobacillus is probably not what happened to you though because it certainly did not leave a flavor of mushroom in my beer. That is, unless your mushroom flavor is sort of sour.
I've only had infections twice in beers in nearly 40 years of brewing, but I believe both were lactobacillus infections. Both were also in the summertime.
