What's your worst experience with "not invented here syndrome"?

In the recent past I've been dealing with custom made JS datepickers, autocomplete boxes and various other widgets that were purpose made for some feature. Almost every single widget the app uses is built from scratch.

Now there are new features that need these widgets to behave differently, and needless to say, none were built with customisation or extendability in mind.

Hardly shocking, I know, but I'm the one that has to spend several hours to get these widgets to work for the new features instead of using some of the many open-source, tested, mature and customisable solutions that are out there.

Myself

Never wanted to use frameworks, now I know better

University project, team of three, Java CLI application. One teammate caused delay after delay because he considered himself smarter than Oracle employees.

"I''ll improve performance with my own HashMap implementation". "Lists are so slow, I'll replace all of them with arrays". And so on. So he did, on his own branch, barely contributing to the actual goal of the project.

When we met after he was done, he started the app with a shit-eating grin. "You'll be amazed". The app ran and did its calculations. And ran... and ran... his grin disappearing from his face bit by bit, getting replaced by a frown.
Yes we were "amazed" - after all his work, the app was exactly as performant as before. Actually using a profiler revealed the hot spots were anywhere but in the parts he optimized.

Later he trash talked the JVM, because when his optimisations don't actually improve performance it's clearly "deadlocks somewhere". Suuure... 🤷‍♂️

On my current work, the guys which created the legacy system (and jump out of the boat) create a very complex and useless api with nodejs for each single thing, even the promises instead they use tested and mature libraries they created their own promises handlers which is a pain in the ass to work out.

And they keep the project without update for 9 years along.

Conclusion, the project reached at a point which is necessary to rewrite whole application because the legacy to add new features it'll gonna broke everything.