Tech investors are a fickle, capricious lot, behind only oil traders and buyers of Chinese equities in their lack of forgiveness.

Shares of a bevy of tech companies have been slammed this week, each one previously a darling to a greater or lesser degree. Their transgressions varied, from downright dreadful results to soft forecasts to unexpected tax snafus. What these falling angels have in common is an investment community that sells first and asks questions later.

It’s a good lesson for newer observers of the tech scene: Yes, in fact the lofty valuations of a year or two ago were ridiculous and unsustainable. To have believed otherwise was naïve.

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Take GoPro (GPRO), a fine company with an innovative product its customers love. GoPro’s problem of late is that its newer wares aren’t so great and it has too few customers. It didn’t help that it lowered first-quarter revenue guidance by about $130 million. GoPro’s is a single-digit stock now, down from more than $65 this summer.

Shares of LinkedIn were hammered in after-hours trading last night, down 29%, after the recruiting site reported a weak forecast for 2016. LinkedIn (LNKD) has a giant business, but it loses money and it is priced for perfection, as the market mavens say. Now it’s priced for less perfection.

Shares of data-analytics software maker Tableau Software (DATA) were hit even harder, despite robust revenue growth. The companies shares were off about 36% after hours after it disclosed it was unlikely to benefit as planned from a deferred tax asset. (Fortune’s Andrew Nusca ferrets out the details here.)

This is how it goes. When times are good, problems are easy to overlook. When times are tougher, as they are now, every foible is punished.