Two instances of construct open. Both had been working fine, just screwing about trying different things. Closed the second one and went back to the first, went to make a runtime and got this error
<img src="http://steamgauge.com/misc/construct/error.jpg">
Happened each time I tried. Restarted it, worked fine again. Can't replicate it, don't know what caused it (I wasn't doing anything complex tho).