Unfortunately the best testing can only test for what you are looking for.
Test scripts will only be based on what are considered possible scenarios.
Furthermore, the IT industry is now full of people who have been taught to program, not learnt to program.
One would like to think an operating system like windows would be fully tested, yet a host of bug-fixes are released every month, for years.
Yawn !
You've already been told above not to compare the generic IT industry to the niche part of the IT industry that deals with safety-critical systems.
Windows and such like are not subjected to the same formal design process that safety critical systems are. Thus of course you will get a greater number of bugs, some of which may cause substantial issues to the stability of the system. Bleeding edge generic IT projects these days can use agile development.....no such thing in safety critical systems.... in safety critical systems there is a traceability requirement for you to be able to show that a given line of source code fulfils given requirement specifications, the level of detail is truly excruciatingly tedious... but it all serves a very important purpose !
Sure there's always scope for issues, but the whole point of the almost obsessive-compulsive development methodology for safety critical systems is that failure or bugs won't kill anyone !