I seem to find that most people upon moving into their new home discover something bizarre or something completely bodged (usually hidden) by the previous owner.
In our first house we found that the garage access door had obviously been broken during move day and as we opened it...it completely fell apart and was covered in still wet glue! 
In the same house we also found the previous owners had lazily carpeted around large pieces of furniture...so two tone carpet in the bedrooms. Cue scrambling around trying to get new carpet fitted asap.
Current house, I assume that the vendors only ever cleaned what could be seen immediately at eye level or used useless cleaning products. The kitchen cupboards were a sticky, dusty mess of 15 years worth of grease and oil. The venetian kitchen blind was also sticky with grease. The sink drain brown with tea stains etc. I had to invest in heavy duty chemical degreaser and a drill brush and spent approx 10 hours scrubbing and breathing chemical fumes the day after moving in. Bizarrely the dirt was not particularly obvious when we viewed, I guess we didn't actually touch anything too much due to covid etc. 
I appreciate the above is probably not as horrific as what some people find, but it always surprises me what people see as "normal" or feel they can get away with.
Anyone else find any horrors worth sharing?