It happens. People come into Colorado Springs to visit and don’t let themselves acclimate if they’re from lower elevations and then they try to do pikes peak, which is about 3k feet lower than Everest base camp. The altitude sickness can be legit.
This was me :) live at about 900' flew in for work, picked up my rental Jeep and drove up to pikes peak
Didn't get sick but could walk like 5 steps at a time before needing to catch my breath. Hung out for a bit came down and went up to Longmont. Had a nice headache for a couple days.
Second trip out I skipped pikes, but also acclimated way faster. No headache, and got in some great hiking.