My own rather short academic career involved doing lab work with three different PI-led groups. One PI was actually excellent, and I really had no idea how good I had it. I caught the other two engaging in deliberately fraudulent practices. For example, data they'd collect from experiments would be thrown out selectively so that they could publish better curve-fits. Another trick was fabricating data with highly obscure methods that other groups would be unlikely to replicate. They'd also apply pressure to graduate students to falsify data in order to get results that agreed with their previously published work.

The main difference between the excellent PI and the two fraudsters was that the former insisted on everyone in her lab keeping highly accurate and detailed daily lab notebooks, while the other two had incredibly poor lab notebook discipline (and often didn't even keep records!). She actually caught one of her grad students fudging data via this method, before it went to publication. Another requirement was that samples had to be blindly randomized before we analyzed them, so that nobody could manipulate the analytical process to get their desired result.

If you're thinking about going into academia, that's the kind of thing to look out for when visiting prospective PIs. Shoddy record keeping is a huge red flag. Inability to replicate results, and in particular no desire to replicate results, is another warning sign. And yes, a fair number of PIs have made careers out of publishing fraudulent results and never get caught, and they infest the academic system.