I have standard (K)Ubuntu system that has Qt 4 installed via packages and Qt Creator as well as Qt 5.1 installed via the online installer. I'm running Qt Creator from there.

The distribution has been upgraded over the years incrementally.

What happens now frequently is that Qt Creator is running out of open file descriptors, and a quick look into /proc/<pid>/fd suggests that Qt Creator forgets to close help file descriptors:

<last few lines of ls -l output from /proc/<pid/fd>
lrwx------ 1 simon simon 64 Jul 24 12:40 97 -> /home/simon/Qt/5.1.0/gcc/doc/qtsql.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 970 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 971 -> /usr/share/qt4/doc/qch/qt.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 972 -> /usr/share/qt4/doc/qch/assistant.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 973 -> /usr/share/qt4/doc/qch/designer.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 974 -> /usr/share/qt4/doc/qch/linguist.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 975 -> /usr/share/qt4/doc/qch/qmake.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 976 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 977 -> /usr/share/qt4/doc/qch/qt.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 978 -> /usr/share/qt4/doc/qch/assistant.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 979 -> /usr/share/qt4/doc/qch/designer.qch
lrwx------ 1 simon simon 64 Jul 24 12:40 98 -> /home/simon/Qt/5.1.0/gcc/doc/qtsvg.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 980 -> /usr/share/qt4/doc/qch/linguist.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 981 -> /usr/share/qt4/doc/qch/qmake.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 982 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 983 -> /usr/share/qt4/doc/qch/qt.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 984 -> /usr/share/qt4/doc/qch/assistant.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 985 -> /usr/share/qt4/doc/qch/designer.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 986 -> /usr/share/qt4/doc/qch/linguist.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 987 -> /usr/share/qt4/doc/qch/qmake.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 988 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 989 -> /usr/share/qt4/doc/qch/qt.qch
lrwx------ 1 simon simon 64 Jul 24 12:40 99 -> /home/simon/Qt/5.1.0/gcc/doc/qttestlib.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 990 -> /usr/share/qt4/doc/qch/qmake.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 991 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 992 -> /usr/share/qt4/doc/qch/qt.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 994 -> /usr/share/qt4/doc/qch/assistant.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 995 -> /usr/share/qt4/doc/qch/designer.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 996 -> /usr/share/qt4/doc/qch/linguist.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 997 -> /usr/share/qt4/doc/qch/qmake.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 998 -> /usr/share/qt4/doc/qch/qml.qch
lr-x------ 1 simon simon 64 Jul 24 12:40 999 -> /usr/share/qt4/doc/qch/qt.qch

In the options I removed old "com.trolltech.assistant.4xxx" now, so that might hopefully do the trick for me. But Erik said I should file a bug report