Oh, how many times have I seen this mistake made? Probably about as many times as I’ve made it myself!
It’s all too easy to do, this is ‘just an upgrade’ or a ‘straightforward change’ or even a ‘simple addition’. The testing is therefore assumed to be minimal, and to take very little time as everything is bound to pass.

Of course, what actually happens is the same for most IT projects – the scope was underestimated and the finished product has more bugs than we thought it would. What was scheduled to take a day is suddenly lasting a week, developers who are needed to fix the bugs have already had their time assigned to other projects, and deadlines are either fast approaching or have already been and gone.

There’s no real remedy to this that I know of outside of pessimism from the outset – this pessimism leads to increased estimates and contingency, although there’s no guarantee that the project will agree to either! Assume that the application you’re going to get will fall over as soon as you even glance at it. Assume that everything will collapse with even the most basic of tests – that way you can only ever be pleasantly surprised.