when tcollect goes (sorta) buggy....

[parisse]

It is unfortunately impossible to predict the time that will be required to perform simplifications, and the Prime OS can currently not catch exceptions, making interruptions by ON difficult to propagate.
As for tcollect, it just does what it should, that is collect cos and sin of the same angle in one trigonometric function by the rule a*cos(theta)+b*sin(theta)=sqrt(a^2+b^2)*cos(theta-theta0)
You can avoid some sign and absolute values by making assumptions on x. But even with an assumption, calling tcollect is not a good idea here, as one can see just looking at the answer. And calling simplify on such an ugly answer is certainly not a good idea!