> I left them exposed for 30 minutes. This is a ridiculous amount of time to leave agar plates open for, since usually you’d only open them for a few seconds.

It's actually really common to let them exposed that long. Not when using them, but to make them: once the agar medium is melted and poured in the petri dishes, closing the lid during cooling causes condensation. Having the petri dish full of water when using it is more difficult and annoying, so it's better to let them cool down with the lid open (a space-efficient way to do so is to have a pyramid of plates where each lid rests on two plates and can support one)

Also, two contaminations out of 4 plates sounds really really bad, but then the blog doesn't say how the agar plates were prepared (how many plates without exposures were contaminated?) and how long they were incubated (sometimes something starts growing after a week or two, if you're culturing a fast-growing bacteria then it's mostly irrelevant).